是`X x = x = X（）;`legal C ++？ [英] Is `X x = x = X();` legal C++?

问题描述

我减少了这个：

struct A
{
   int * x;
   A() : x( x = new int() )
   {
   }
};

：

int m = m = 3;
//or
struct X;
//...
X x = x = X();

似乎对我合法。我不明白你为什么要这样做，但它是合法的吗？有没有情况下，你想这样做（不是 int 情况，我意识到这是完全无用的）？

Seems legal to me. I don't see why you'd want to do it, but is it legal? Are there cases where you'd want to do this (not the int case, I realize that's completely useless)?

推荐答案

这取决于你如何定义法律。它将编译;

It depends on how you define "legal". It will compile; that doesn't mean that it is guaranteed to work.

直到完整语句 X x = ... 执行， x 未初始化。它不是 X 。因此，执行 x = X（）表示创建一个临时 X 并调用 X： ： x 。

Until the full statement X x = ... executes, x is uninitialized. It is not an X yet. Therefore, performing x = X() means to create a temporary X and call X::operator=(const X&) on the uninitialized variable x.

在非 POD类实例尚未初始化（谁的构造函数尚未被调用）产生未定义的行为。如果 X 是POD类型（或在C ++ 11中很简单），那么它将工作。但否则没有。

Calling a function on a non-POD class instance that has not been initialized (who's constructor has not yet been called) yields undefined behavior. If X is a POD type (or trivial in C++11), then it will work. But otherwise no.

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