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);