[英]Should a constructor ever be called on assignment?
The output of the code below in VS2015 is "constructor". VS2015中以下代码的输出是“构造函数”。
Shouldn't it fail to compile due to the missing assignment operator? 由于缺少赋值运算符,它不应该无法编译吗?
struct A { };
struct B {
B(){}
B(const A& a) {
cout << "constructor" << endl;
}
//B& operator=(const A& a) {
// cout << "assignment operator" << endl;
// return *this;
//}
};
int main() {
A a;
B b;
b = a;
return 0;
}
Yes, when there is a conversion going on, like in your testcase. 是的,当有转换时,就像在您的测试用例中一样。
You're effectively calling 你有效地打电话了
b = B(a);
Because B
's assignment operator B& operator=(B const&)
is implicitly declared, it is found during overload resolution. 因为
B
的赋值运算符B& operator=(B const&)
是隐式声明的,所以在重载解析期间会找到它。 Because your assignment is only one conversion away from being a match (and that's exactly the number of conversions that are allowed to happen), it converts a
to B
and then assigns the new temporary B
to b
. 因为您的分配只是一次转换而不是匹配(并且这恰好是允许发生的转换次数),所以它将
a
转换为B
,然后将新的临时B
分配给b
。
Let's consider an analogous example. 让我们考虑一个类似的例子。
double a;
int b=5;
a=b;
The only thing you can assign to a double
is another double
. 你可以分配给
double
的唯一的东西是另一个double
。 However, an int
can be converted to a double
. 但是,
int
可以转换为double
。
Similarly here, A
can be converted to B
, because the constructor to do so exists. 同样在这里,
A
可以转换为B
,因为存在这样的构造函数。 And that's what happens. 这就是发生的事情。
Your code has implicit conversion from A
to B
, the b = a
is compiled as b = B(a);
你的代码有从
A
到B
隐式转换, b = a
编译为b = B(a);
. 。 If you want this to be detected as error, you can use the
explicit
specifier : 如果要将其检测为错误,可以使用
explicit
说明符 :
struct B {
B(){}
explicit B(const A& a) {
std::cout << "constructor" << std::endl;
}
};
Then you should get error, such as these generated by ideone.com : 然后你应该得到错误,例如ideone.com生成的错误 :
prog.cpp: In function 'int main()':
prog.cpp:20:7: error: no match for 'operator=' (operand types are 'B' and 'A')
b = a;
^
prog.cpp:5:8: note: candidate: B& B::operator=(const B&)
struct B {
^
prog.cpp:5:8: note: no known conversion for argument 1 from 'A' to 'const B&'
prog.cpp:5:8: note: candidate: B& B::operator=(B&&)
prog.cpp:5:8: note: no known conversion for argument 1 from 'A' to 'B&&'
After that, the constructor will never be implicitly called, and if you want to call it, you have to write it explicitly: b = B(a);
之后,永远不会隐式调用构造函数,如果要调用它,则必须明确地编写它:
b = B(a);
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