默认赋值运算符检查自赋值

Question

我想知道赋值运算符的默认实现是否检查自赋值，所以这两个实现中的哪一个可以被认为最接近默认实现：

class A{
    int x;
public :
    ...
    // first one
    A& operator=(const A& a){
        if(this != &a) x = a.x;
        return *this;
    }
    // second one
    A& operator=(const A& a){
        x = a.x;
        return *this;
    }
}

我搜索了 C++ 标准，但我能找到的唯一一个是 this，但没有关于此的任何内容

Answer 1

不，实现不检查 "self":

https://en.wikipedia.org/wiki/Assignment_operator_(C%2B%2B)

The copy assignment operator, often just called the "assignment operator", is a special case of assignment operator where the source (right-hand side) and destination (left-hand side) are of the same class type.

It is one of the special member functions, which means that a default version of it is generated automatically by the compiler if the programmer does not declare one.

The default version performs a memberwise copy, where each member is copied by its own copy assignment operator (which may also be programmer-declared or compiler-generated).

Answer 2

赋值运算符不检查自赋值。因此，您的第二个实现最接近默认实现。

我在标准中看不到关于任何此类优化的任何文字，如果我的 class 的属性在某些情况下不会由编译器生成的运算符分配，那将很奇怪。想象一些属性分配是用户定义的并且执行一些不常见的任务。编译器不知道这一点，即使我为自己分配了一个对象，IMO 也应该调用它们。

默认赋值运算符检查自赋值

Default assignment operator checks for self assignment

c++

standards

operators