explicit关键字用于取消构造函数的隐式转换，对有多个参数的构造函数使用explicit是个语法错误。
In C++ it is possible to declare constructors for a class, taking a
single parameter, and use those constructors for doing type conversion.
For example:
class A {
public:
A(int);
};
void f(A) {}
void g()
{
A a1 = 37;
A a2 = A(47);
A a3(57);
a1 = 67;
f(77);
}
A declaration like:
A a1 = 37;
says to call the A(int) constructor to create an A object from
the integer value. Such a constructor is called a "converting
constructor".
However, this type of implicit conversion can
be confusing, and there is a way of disabling it, using a new keyword
"explicit" in the constructor declaration:
class A {
public:
explicit A(int);
};
void f(A) {}
void g()
{
A a1 = 37; // illegal
A a2 = A(47); // OK
A a3(57); // OK
a1 = 67; // illegal
f(77); // illegal
}
Using the explicit keyword, a constructor is declared to be
"nonconverting", and explicit constructor syntax is required:
class A {
public:
explicit A(int);
};
void f(A) {}
void g()
{
A a1 = A(37);
A a2 = A(47);
A a3(57);
a1 = A(67);
f(A(77));
}
Note that an expression such as:
A(47)
is closely related to function-style casts supported by C++. For example:
double d = 12.34;
int i = int(d);