这一块主要是讲move语义的,我认为这是在C++0x中,最好的特性之一,因为它几乎可以完全透明的提高效率。
在Stephan T. Lavavej这篇帖子之后,有很多评论,大体上认为C++因为这些特性而变得更复杂了,而难以掌握,另初学者望而生畏。
但是我认为这是值得的,因为C++的宗旨是:“don't pay for what you don't use 不要为你不使用的东西而付出代价”。
由于水平有限,错误之处,请多多指教:
move semantics: the pattern
move语义:模式
Here's a simple class, remote_integer, that stores a pointer to a dynamically allocated int . (This is "remote ownership".) Its default constructor, unary constructor, copy constructor, copy assignment operator, and destructor should all look very familiar to you. I've additionally given it a move constructor and move assignment operator. They're guarded by #ifdef MOVABLE so that I can demonstrate what happens with and without them; real code won't do this.
下面是一个简单的类remote_integer, 这个类存储了指向动态分配的int的指针(“远程拥有权”)。你对这个类的默认构造函数,一元构造函数,拷贝构造函数,重载的赋值运算和析构函数都很熟悉。我给它增加了move构造函数和move 赋值运算。它们被#ifdef MOVABLE 保护,所以我可以演示在没有move构造函数和move 赋值运算时会发生什么,真正的代码是没有这个地。
1
//C:\Temp>type remote.cpp
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#include <stddef.h>
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#include <iostream>
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#include <ostream>
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using namespace std;
10![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
11
12![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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class remote_integer
{
14![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
15
public:
16![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
17![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer()
{
18![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
19
cout << "Default constructor." << endl;
20![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
21
22![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = NULL;
24![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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}
26![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
27
28![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
29![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
explicit remote_integer(const int n)
{
30![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
31
cout << "Unary constructor." << endl;
32![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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34![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = new int(n);
36![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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}
38![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
39
40![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
41![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(const remote_integer& other)
{
42![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
43
cout << "Copy constructor." << endl;
44![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
45
46![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
47![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
48![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
49
m_p = new int(*other.m_p);
50![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
51![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
52![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = NULL;
54![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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}
56![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
57
}
58![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
59
60![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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#ifdef MOVABLE
62![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
63![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(remote_integer&& other)
{
64![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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cout << "MOVE CONSTRUCTOR." << endl;
66![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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68![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = other.m_p;
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other.m_p = NULL;
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}
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75
#endif // #ifdef MOVABLE
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77
78![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
79![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(const remote_integer& other)
{
80![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
81
cout << "Copy assignment operator." << endl;
82![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
83
84![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
85![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
86![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
87
delete m_p;
88![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
89
90![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
91![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
92![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
93
m_p = new int(*other.m_p);
94![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
95![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
96![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = NULL;
98![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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}
100![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
101
}
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return *this;
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107
}
108![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
109
110![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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#ifdef MOVABLE
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remote_integer& operator=(remote_integer&& other)
{
114![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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cout << "MOVE ASSIGNMENT OPERATOR." << endl;
116![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
117
118![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
119![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
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delete m_p;
122![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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124![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = other.m_p;
126![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
127
other.m_p = NULL;
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129
}
130![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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132![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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return *this;
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135
}
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#endif // #ifdef MOVABLE
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139
140![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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~remote_integer()
{
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143
cout << "Destructor." << endl;
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145
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delete m_p;
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}
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151
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int get() const
{
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return m_p ? *m_p : 0;
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157
}
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private:
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int * m_p;
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};
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167
168![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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remote_integer square(const remote_integer& r)
{
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const int i = r.get();
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return remote_integer(i * i);
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}
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int main()
{
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remote_integer a(8);
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cout << a.get() << endl;
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remote_integer b(10);
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cout << b.get() << endl;
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b = square(a);
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cout << b.get() << endl;
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}
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//C:\Temp>cl /EHsc /nologo /W4 remote.cpp
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//remote.cpp
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//C:\Temp>remote
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Unary constructor.
218![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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8
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Unary constructor.
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10
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Unary constructor.
226![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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Copy assignment operator.
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Destructor.
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64
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Destructor.
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Destructor.
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//C:\Temp>cl /EHsc /nologo /W4 /DMOVABLE //remote.cpp
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//remote.cpp
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//C:\Temp>remote
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Unary constructor.
248![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
249
8
250![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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Unary constructor.
252![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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10
254![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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Unary constructor.
256![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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MOVE ASSIGNMENT OPERATOR.
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Destructor.
260![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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64
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Destructor.
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Destructor.
266![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
267![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
There are several things to notice here.
这里有几点需要注意:
· The copy and move constructors are overloaded, and the copy and move assignment operators are overloaded. We've already seen what happens to functions overloaded on const Type& and Type&& . This is what allows b = square(a); to automatically select the move assignment operator when it's available.
拷贝构造函数和move构造函数式重载版本,赋值操作和move赋值操作也是重载版本,我们已经看到当函数通过const Type&和Type&&进行重载时,结果的不同。当move语义可用时,b=square(a)将会选择move赋值操作。
· Instead of dynamically allocating memory, the move constructor and move assignment operator simply steal it from other . When stealing, we copy other's pointer and then null it out. When other is destroyed, its destructor will do nothing.
move构造函数和move赋值操作只是简单的将内存从other那里“偷”了过来,而不是重新分配新的内存。当我们“偷”内存时,我们只是复制other的指向内存的指针,然后other的指针置为null。当other被摧毁时,析构函数什么都不用干(不用delete)。
· Both the copy and move assignment operators need self-assignment checks. It's well-known why copy assignment operators need self-assignment checks. This is because plain old data types like ints can be assigned to themselves harmlessly (e.g. with x = x;), so user-defined data types should also be harmlessly self-assignable. Self-assignment virtually never happens in handwritten code, but it can easily happen inside algorithms like std::sort() . In C++0x, algorithms like std::sort() can move elements around instead of copying them. The same potential for self-assignment exists here.
普通赋值操作和move赋值操作都需要进行自我赋值检查,赋值操作需要进行自我赋值检查是广为人知的。这是因为像int这种内建类型可以正确的自我赋值(例如:x=x),所以,用户的自定义类型也应该可以正确的进行自我赋值,自我赋值实际上在手写代码里面是不存在的,但是在类似std::sort()之类的算法中,却很常见。在C++0x中,像std::sort()之类的算法使用了move语义而非拷贝语义,所以同样的move赋值操作也需要进行自我赋值检查。
At this point, you may be wondering how this interacts with automatically generated ("implicitly declared" in Standardese) constructors and assignment operators.
这样,你可能会想move拷贝构造函数和move赋值操作和编译器自动生成拷贝构造函数和赋值操作之间是什么关系呢,会不会自动生成move的呢。
· Move constructors and move assignment operators are never implicitly declared.
Move构造函数和move赋值操作都不会自动生成。
· The implicit declaration of a default constructor is inhibited by any user-declared constructors, including copy constructors and move constructors.
如果用户声明了构造函数,包括拷贝构造函数和move构造函数,默认构造函数将不会自动生成。
· The implicit declaration of a copy constructor is inhibited by a user-declared copy constructor, but not a user-declared move constructor.
用户定义了拷贝构造函数以后,编译器将不会再自动生成拷贝构造函数,但是move构造函数例外。
· The implicit declaration of a copy assignment operator is inhibited by a user-declared copy assignment operator, but not a user-declared move assignment operator.
用户定义了赋值操作以后,编译器将不会再自动生成赋值操作,但是move赋值操作例外。
Basically, the automatic generation rules don't interact with move semantics, except that declaring a move constructor, like declaring any constructor, inhibits the implicitly declared default constructor.
基本上,除了move构造函数会阻止自动生成默认构造函数以外,自动生成规则不影响move语义。
move semantics: moving from lvalues
move 语义:从左值move
Now, what if you like to write your copy constructors in terms of your copy assignment operators? You might attempt to write your move constructors in terms of your move assignment operators. This is possible, but you have to be careful. Here's the wrong way to do it:
现在,如果你在拷贝构造函数中使用赋值操作会怎么样呢?你可能会在move构造函数中使用你的move赋值操作。这是可能的,但是你必须小心。下面是一种错误的使用方法:
1
//C:\Temp>type unified_wrong.cpp
2![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
3
#include <stddef.h>
4![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
5
#include <iostream>
6![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
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#include <ostream>
8![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
9
using namespace std;
10![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
11
12![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
13![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class remote_integer
{
14![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
15
public:
16![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
17![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer()
{
18![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
19
cout << "Default constructor." << endl;
20![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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22![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = NULL;
24![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
25
}
26![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
27
28![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
29![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
explicit remote_integer(const int n)
{
30![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
31
cout << "Unary constructor." << endl;
32![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
33
34![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = new int(n);
36![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
37
}
38![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
39
40![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
41![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(const remote_integer& other)
{
42![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
43
cout << "Copy constructor." << endl;
44![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
45
46![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
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m_p = NULL;
48![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
49
*this = other;
50![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
51
}
52![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
53
54![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
55
#ifdef MOVABLE
56![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
57![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(remote_integer&& other)
{
58![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
59
cout << "MOVE CONSTRUCTOR." << endl;
60![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
61
62![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
63
m_p = NULL;
64![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
65
*this = other; // WRONG
66![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
67
}
68![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
69
#endif // #ifdef MOVABLE
70![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
71
72![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
73![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(const remote_integer& other)
{
74![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
75
cout << "Copy assignment operator." << endl;
76![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
77
78![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
79![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
80![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
81
delete m_p;
82![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
83
84![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
85![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
86![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
87
m_p = new int(*other.m_p);
88![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
89![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
90![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
91
m_p = NULL;
92![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
93
}
94![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
95
}
96![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
97
98![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
99
return *this;
100![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
101
}
102![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
103
104![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
105
#ifdef MOVABLE
106![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
107![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(remote_integer&& other)
{
108![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
109
cout << "MOVE ASSIGNMENT OPERATOR." << endl;
110![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
111
112![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
113![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
114![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
115
delete m_p;
116![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
117
118![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
119
m_p = other.m_p;
120![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
121
other.m_p = NULL;
122![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
123
}
124![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
125
126![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
127
return *this;
128![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
129
}
130![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
131
#endif // #ifdef MOVABLE
132![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
133
134![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
135![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
~remote_integer()
{
136![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
137
cout << "Destructor." << endl;
138![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
139
140![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
141
delete m_p;
142![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
143
}
144![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
145
146![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
147![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int get() const
{
148![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
149
return m_p ? *m_p : 0;
150![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
151
}
152![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
153
154![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
155
private:
156![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
157
int * m_p;
158![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
159
};
160![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
161
162![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
163![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
remote_integer frumple(const int n)
{
164![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
165![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (n == 1729)
{
166![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
167
return remote_integer(1729);
168![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
169
}
170![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
171
172![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
173
remote_integer ret(n * n);
174![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
175
176![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
177
return ret;
178![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
179
}
180![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
181
182![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
183![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
184![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
185
remote_integer x = frumple(5);
186![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
187
188![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
189
cout << x.get() << endl;
190![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
191
192![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
193
remote_integer y = frumple(1729);
194![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
195
196![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
197
cout << y.get() << endl;
198![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
199
}
200![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
201
202![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
203
//C:\Temp>cl /EHsc /nologo /W4 /O2 unified_wrong.cpp
204![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
205
//unified_wrong.cpp
206![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
207
208![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
209
//C:\Temp>unified_wrong
210![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
211
Unary constructor.
212![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
213
Copy constructor.
214![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
215
Copy assignment operator.
216![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
217
Destructor.
218![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
219
25
220![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
221
Unary constructor.
222![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
223
1729
224![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
225
Destructor.
226![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
227
Destructor.
228![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
229
230![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
231
//C:\Temp>cl /EHsc /nologo /W4 /O2 /DMOVABLE unified_wrong.cpp
232![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
233
//unified_wrong.cpp
234![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
235
236![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
237
//C:\Temp>unified_wrong
238![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
239
Unary constructor.
240![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
241
MOVE CONSTRUCTOR.
242![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
243
Copy assignment operator.
244![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
245
Destructor.
246![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
247
25
248![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
249
Unary constructor.
250![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
251
1729
252![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
253
Destructor.
254![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
255
Destructor.
256![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
257![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
(The compiler is performing the RVO here, but not the NRVO. As I mentioned earlier, some copy constructor calls are elided by the RVO and NRVO, but the compiler isn't always able to apply them. Move constructors optimize the remaining cases.)
(编译器进行了返回值优化RVO,但不是具名返回值优化NRVO。之前提到,有些拷贝构造函数被RVO或NRVO优化掉了,但是并不是所有的情况下都能优化掉,move构造函数可以优化剩下的情况。)
The line marked WRONG inside the move constructor is calling the copy assignment operator! This compiles and runs, but it defeats the purpose of the move constructor.
在有WRONG的那一行,在move构造函数中调用了赋值操作,编译通过了,但结果却并不对(因为那个赋值操作调用的是普通的赋值操作)。
What happened? Remember from C++98/03 that named lvalue references are lvalues (if you say int& r = *p; then r is an lvalue) and unnamed lvalue references are also lvalues (given vector<int> v(10, 1729), calling v[0] returns int& , an unnamed lvalue reference whose address you can take). Rvalue references behave differently:
为什么呢?在C++98/03中,具名的左值引用是左值,不具名的左值引用还是左值。但是右值引用却不同:
· Named rvalue references are lvalues.
具名的右值引用是左值。
· Unnamed rvalue references are rvalues.
不具名的右值引用才是右值。
A named rvalue reference is an lvalue because it can be repeatedly mentioned, with multiple operations performed on it. If instead it were an rvalue, then the first operation performed could steal from it, affecting subsequent operations. Stealing is supposed to be unobservable, so this is forbidden. On the other hand, unnamed rvalue references can't be repeatedly mentioned, so they can preserve their rvalueness.
具名的右值引用是左值的原因在于它可以被重复的使用,对它进行各种各样的操作。如果它还是右值的话,在第一个操作中被“偷”走了,后面的操作将会受到影响。“偷”应该不被察觉到(应该和没有“偷”时表现一样),所以不能这样做。另一方面,不具名的右值引用不能被重复的使用,所以它们仍然是右值。
If you're really intent on implementing your move constructors in terms of your move assignment operators, you'll need the ability to move from lvalues by treating them as rvalues. This is powered by std::move() from C++0x <utility>, which will be in VC10 (in fact, it's already in my development build), but because it's not in the VC10 CTP, I'll show you how to write it from scratch:
如果你确实想在move构造函数中使用move赋值操作,你需要把左值当做右值对待。这可以通过使用C++0x <utility>中的std::move()来解决。VC10TCP版本中没有,但是VC10中会有,所以我会教你从头做起:
1
//C:\Temp>type unified_right.cpp
2![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
3
#include <stddef.h>
4![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
5
#include <iostream>
6![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
7
#include <ostream>
8![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
9
using namespace std;
10![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
11
12![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
13![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference
{
14![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
15
typedef T type;
16![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
17
};
18![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
19
20![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
21![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference<T&>
{
22![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
23
typedef T type;
24![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
25
};
26![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
27
28![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
29![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference<T&&>
{
30![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
31
typedef T type;
32![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
33
};
34![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
35
36![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
37![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> typename RemoveReference<T>::type&& Move(T&& t)
{
38![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
39
return t;
40![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
41
}
42![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
43
44![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
45![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class remote_integer
{
46![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
47
public:
48![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
49![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer()
{
50![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
51
cout << "Default constructor." << endl;
52![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
53
54![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
55
m_p = NULL;
56![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
57
}
58![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
59
60![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
61![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
explicit remote_integer(const int n)
{
62![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
63
cout << "Unary constructor." << endl;
64![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
65
66![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
67
m_p = new int(n);
68![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
69
}
70![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
71
72![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
73![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(const remote_integer& other)
{
74![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
75
cout << "Copy constructor." << endl;
76![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
77
78![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
79
m_p = NULL;
80![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
81
*this = other;
82![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
83
}
84![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
85
86![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
87
#ifdef MOVABLE
88![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
89![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(remote_integer&& other)
{
90![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
91
cout << "MOVE CONSTRUCTOR." << endl;
92![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
93
94![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
95
m_p = NULL;
96![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
97
*this = Move(other); // RIGHT
98![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
99
}
100![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
101
#endif // #ifdef MOVABLE
102![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
103
104![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
105![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(const remote_integer& other)
{
106![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
107
cout << "Copy assignment operator." << endl;
108![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
109
110![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
111![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
112![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
113
delete m_p;
114![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
115
116![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
117![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
118![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
119
m_p = new int(*other.m_p);
120![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
121![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
122![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
123
m_p = NULL;
124![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
125
}
126![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
127
}
128![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
129
130![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
131
return *this;
132![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
133
}
134![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
135
136![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
137
#ifdef MOVABLE
138![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
139![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(remote_integer&& other)
{
140![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
141
cout << "MOVE ASSIGNMENT OPERATOR." << endl;
142![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
143
144![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
145![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
146![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
147
delete m_p;
148![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
149
150![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
151
m_p = other.m_p;
152![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
153
other.m_p = NULL;
154![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
155
}
156![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
157
158![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
159
return *this;
160![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
161
}
162![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
163
#endif // #ifdef MOVABLE
164![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
165
166![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
167![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
~remote_integer()
{
168![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
169
cout << "Destructor." << endl;
170![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
171
172![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
173
delete m_p;
174![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
175
}
176![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
177
178![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
179![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int get() const
{
180![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
181
return m_p ? *m_p : 0;
182![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
183
}
184![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
185
186![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
187
private:
188![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
189
int * m_p;
190![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
191
};
192![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
193
194![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
195![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
remote_integer frumple(const int n)
{
196![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
197![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (n == 1729)
{
198![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
199
return remote_integer(1729);
200![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
201
}
202![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
203
204![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
205
remote_integer ret(n * n);
206![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
207
208![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
209
return ret;
210![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
211
}
212![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
213
214![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
215![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
216![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
217
remote_integer x = frumple(5);
218![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
219
220![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
221
cout << x.get() << endl;
222![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
223
224![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
225
remote_integer y = frumple(1729);
226![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
227
228![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
229
cout << y.get() << endl;
230![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
231
}
232![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
233
234![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
235
//C:\Temp>cl /EHsc /nologo /W4 /O2 /DMOVABLE unified_right.cpp
236![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
237
//unified_right.cpp
238![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
239
240![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
241
//C:\Temp>unified_right
242![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
243
Unary constructor.
244![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
245
MOVE CONSTRUCTOR.
246![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
247
MOVE ASSIGNMENT OPERATOR.
248![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
249
Destructor.
250![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
251
25
252![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
253
Unary constructor.
254![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
255
1729
256![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
257
Destructor.
258![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
259
Destructor.
260![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
261![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
(I'll refer to std::move() and my Move() interchangeably, since they're implemented identically.) How does std::move() work? For the moment, I'm going to say that "magic is involved". (There's a full explanation below; it's not complicated, but it involves template argument deduction and reference collapsing, which we'll encounter while looking at perfect forwarding.) I can skip over the magic with concrete examples. Given an lvalue of type string, like up from the overload resolution examples above, std::move(up) calls string&& std::move(string&) . This returns an unnamed rvalue reference, which is an rvalue. Given an rvalue of type string, like strange() from above, std::move(strange()) calls string&& std::move(string&&) . Again, this returns an unnamed rvalue reference, which is an rvalue.
(我将交替的使用std::move和Move,因为它们的实现时一样的)它是怎样工作的呢?现在,我要说“这是个魔法”(后面将会有完整的解释,并不是很复杂,但是它与模板参数演绎和引用合并有关,在讲完美转发的时候我们还会遇到)。我可以用一个具体的例子来略过这个“魔法”:给定一个string类型的左值,像前面例子中的up,std::move(up)调用 string&& std::move(string&), 这个函数返回一个不具名的右值引用,也就是右值。给定一个string类型的右值,像前面例子中的strange(), std::(strange()) 调用 string&& std::move(string&&),这个函数还是返回一个不具名的右值,还是右值。
std::move() is useful in other places than implementing move constructors in terms of move assignment operators. Whenever you reach a point where you can say "I have an lvalue here, and its value is going to cease to matter" (e.g. because it's going to be destroyed or assigned to), you can write std::move(your lvalue expression) in order to activate move semantics.
std::move除了能实现move构造函数中是呀move赋值操作以外,还可以在其他地方使用。无论在任何时候,你说“我有一个左值,但是它的值已经不再重要了”,这时你可以使用std::move(你的左值)来使用move语义。
move semantics: movable members
move语义:movable成员
C++0x's Standard classes (e.g. vector, string, regex) have move constructors and move assignment operators, and we've seen how to implement them in our own classes that manually manage resources (e.g. remote_integer). But what about classes containing movable data members (e.g. vector, string, regex, remote_integer)? The compiler won't automatically generate move constructors and move assignment operators for us. So, we'll need to write them ourselves. Fortunately, with std::move() , this is extremely easy:
C++0x标准类(像 vector, string, regex) 都实现了move构造函数和move赋值操作,而且我们已经学习了怎样在我们自己的类中实现它们来手动管理资源(像 remote_integer). 但是当类中含有movable成员变量是,会怎样呢 (像 vector, string, regex, remote_integer)? 编译器不会自动生成move构造函数和move赋值操作。所以,我们要自己实现,幸运的是我们有了std::move后,实现起来非常简单:
1
//C:\Temp>type point.cpp
2![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
3
#include <stddef.h>
4![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
5
#include <iostream>
6![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
7
#include <ostream>
8![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
9
using namespace std;
10![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
11
12![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
13![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference
{
14![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
15
typedef T type;
16![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
17
};
18![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
19
20![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
21![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference<T&>
{
22![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
23
typedef T type;
24![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
25
};
26![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
27
28![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
29![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> struct RemoveReference<T&&>
{
30![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
31
typedef T type;
32![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
33
};
34![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
35
36![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
37![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
template <typename T> typename RemoveReference<T>::type&& Move(T&& t)
{
38![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
39
return t;
40![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
41
}
42![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
43
44![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
45![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class remote_integer
{
46![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
47
public:
48![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
49![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer()
{
50![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
51
cout << "Default constructor." << endl;
52![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
53
54![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
55
m_p = NULL;
56![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
57
}
58![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
59
60![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
61![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
explicit remote_integer(const int n)
{
62![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
63
cout << "Unary constructor." << endl;
64![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
65
66![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
67
m_p = new int(n);
68![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
69
}
70![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
71
72![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
73![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(const remote_integer& other)
{
74![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
75
cout << "Copy constructor." << endl;
76![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
77
78![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
79![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
80![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
81
m_p = new int(*other.m_p);
82![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
83![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
84![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
85
m_p = NULL;
86![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
87
}
88![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
89
}
90![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
91
92![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
93![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer(remote_integer&& other)
{
94![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
95
cout << "MOVE CONSTRUCTOR." << endl;
96![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
97
98![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
99
m_p = other.m_p;
100![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
101
other.m_p = NULL;
102![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
103
}
104![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
105
106![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
107![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(const remote_integer& other)
{
108![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
109
cout << "Copy assignment operator." << endl;
110![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
111
112![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
113![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
114![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
115
delete m_p;
116![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
117
118![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
119![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (other.m_p)
{
120![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
121
m_p = new int(*other.m_p);
122![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
123![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
} else
{
124![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
125
m_p = NULL;
126![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
127
}
128![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
129
}
130![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
131
132![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
133
return *this;
134![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
135
}
136![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
137
138![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
139![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_integer& operator=(remote_integer&& other)
{
140![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
141
cout << "MOVE ASSIGNMENT OPERATOR." << endl;
142![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
143
144![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
145![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (this != &other)
{
146![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
147
delete m_p;
148![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
149
150![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
151
m_p = other.m_p;
152![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
153
other.m_p = NULL;
154![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
155
}
156![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
157
158![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
159
return *this;
160![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
161
}
162![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
163
164![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
165![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
~remote_integer()
{
166![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
167
cout << "Destructor." << endl;
168![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
169
170![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
171
delete m_p;
172![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
173
}
174![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
175
176![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
177![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int get() const
{
178![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
179
return m_p ? *m_p : 0;
180![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
181
}
182![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
183
184![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
185
private:
186![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
187
int * m_p;
188![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
189
};
190![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
191
192![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
193![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class remote_point
{
194![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
195
public:
196![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
197
remote_point(const int x_arg, const int y_arg)
198![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
199![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
: m_x(x_arg), m_y(y_arg)
{ }
200![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
201
202![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
203
remote_point(remote_point&& other)
204![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
205
: m_x(Move(other.m_x)),
206![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
207![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
m_y(Move(other.m_y))
{ }
208![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
209
210![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
211![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
remote_point& operator=(remote_point&& other)
{
212![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
213
m_x = Move(other.m_x);
214![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
215
m_y = Move(other.m_y);
216![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
217
return *this;
218![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
219
}
220![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
221
222![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
223![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int x() const
{ return m_x.get(); }
224![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
225![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int y() const
{ return m_y.get(); }
226![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
227
228![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
229
private:
230![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
231
remote_integer m_x;
232![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
233
remote_integer m_y;
234![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
235
};
236![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
237
238![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
239![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
remote_point five_by_five()
{
240![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
241
return remote_point(5, 5);
242![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
243
}
244![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
245
246![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
247![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
remote_point taxicab(const int n)
{
248![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
249![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
if (n == 0)
{
250![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
251
return remote_point(1, 1728);
252![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
253
}
254![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
255
256![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
257
remote_point ret(729, 1000);
258![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
259
260![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
261
return ret;
262![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
263
}
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int main()
{
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remote_point p = taxicab(43112609);
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cout << "(" << p.x() << ", " << p.y() << ")" << endl;
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p = five_by_five();
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cout << "(" << p.x() << ", " << p.y() << ")" << endl;
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}
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//C:\Temp>cl /EHsc /nologo /W4 /O2 point.cpp
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//point.cpp
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//C:\Temp>point
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Unary constructor.
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Unary constructor.
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MOVE CONSTRUCTOR.
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MOVE CONSTRUCTOR.
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Destructor.
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Destructor.
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(729, 1000)
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Unary constructor.
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Unary constructor.
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MOVE ASSIGNMENT OPERATOR.
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MOVE ASSIGNMENT OPERATOR.
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Destructor.
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Destructor.
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(5, 5)
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Destructor.
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Destructor.
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As you can see, memberwise moves are trivial to write. Note that remote_point's move assignment operator doesn't need to check for self-assignment because remote_integer already does. Also note that remote_point's implicitly declared copy constructor, copy assignment operator, and destructor do the right things.
和你看到的一样,move成员是很无聊的。注意remote_point的move赋值操作没有进行自我赋值检查,因为remote_integer已经检查过了。同样也注意到remote_point自动生成的拷贝构造函数,赋值操作和析构函数都工作正常。
A final reminder: whenever possible, you should implement move constructors and move assignment operators for your copyable classes, because the compiler won't do it for you. Not only will ordinary use of those classes automatically pick up move semantics, but STL containers and algorithms will also take advantage of move semantics, replacing expensive copies with cheap moves.
最后提示,无论在任何时候,你都应该为copyable的类实现move构造函数和move赋值函数,因为编译器不会自动生成。并不只是因为move语义会在通常情况下被使用,而是STL容器和算法会从move语义中得到好处:用简单的move代替昂贵的拷贝。
posted on 2009-05-28 20:51
尹东斐 阅读(1565)
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