c++ - 复制构造函数调用方法

Question

#include<iostream>
using namespace std ; 
class Foo{
    int a , b ;
public:
    Foo(int x, int y){
        a = x ;
        b = y  ;
    }
    Foo(Foo& obj){
        a = obj.a ;
        b = obj.b ;
    }
};

int main(){
    Foo obj(2,3) ;
    Foo obj1(obj) ;
    Foo obj2 = obj ;
}

如果我是对的，Foo obj2 = obj ; 和 Foo obj1(obj) ; 都会调用复制构造函数。

使用其中一个的优点和缺点是什么？

Answer 1

Foo obj1(obj);是直接初始化，Foo obj2 = obj;是拷贝初始化。

这里唯一的区别是第一个可以调用显式构造函数而第二个不能。

但是，没有一个理智的人会让复制构造函数显式，所以假设您正在使用理智的代码，它应该没有什么区别。具有显式复制构造函数的类不满足 CopyConstructible 要求(许多标准容器操作需要)。函数返回和参数传递都使用复制初始化，因此具有显式复制构造函数的类在这些上下文中不可用。

---

只是为了让事情变得清晰。 N3936 §8.5 [dcl.init]/p15-16:

15 The initialization that occurs in the form

T x = a;

as well as in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), and aggregate member initialization (8.5.1) is called copy-initialization. [ Note: Copy-initialization may invoke a move (12.8). —end note ]

16 The initialization that occurs in the forms

T x(a);
T x{a};

as well as in new expressions (5.3.4), static_cast expressions (5.2.9), functional notation type conversions (5.2.3), and base and member initializers (12.6.2) is called direct-initialization.