是否有正确的方法通过C ++中的引用返回一个新的对象实例？ [英] Is there a right way to return a new object instance by reference in C++?

问题描述

所以我写了一些代码，我有这样的：

So I was writing some code, and I had something like this:

class Box
{
    private:
    float x, y, w, h;

    public:
    //...
    Rectangle & GetRect( void ) const
    {
        return Rectangle( x, y, w, h );
    }
};

稍后在一些代码中：

Rectangle rect = theBox.GetRect();

这在我的调试生成中有效，但在发布中有问题 - 我基本上有一个未初始化的矩形。 Rectangle类具有=运算符和复制构造函数。没有深入到为什么这会爆发，我实际上更感兴趣的是通过引用返回一个（新的）对象的正确方法，用于分配复制到一个变量。我只是傻吗？是不是应该做呢？我知道我可以返回一个指针，然后解除引用赋值，但我宁愿不。我的某些部分感觉像通过价值返回会导致对象的冗余复制 - 编译器计算出来并优化它吗？

Which worked in my debug build, but in release there were "issues" returning that Rectangle by reference -- I basically got an uninitialized rectangle. The Rectangle class has an = operator and a copy constructor. Without getting into why this broke, I'm actually more interested in the correct way to return a (new) object by reference for the purpose of assigning copying to a variable. Am I just being silly? Should it not be done? I know I can return a pointer and then dereference on assignment, but I'd rather not. Some part of me feels like returning by value would result in redundant copying of the object -- does the compiler figure that out and optimize it?

这看起来像一个小问题。我感觉几乎尴尬我不知道这多年的C ++编码，所以希望有人可以清除这对我来说。 ：）

It seems like a trivial question. I feel almost embarrassed I don't know this after many years of C++ coding so hopefully someone can clear this up for me. :)

推荐答案

您不能返回对栈上临时对象的引用。您有三个选项：

You can't return a reference to a temporary object on the stack. You have three options:

1. 按值返回




3. 通过引用返回通过引用接收的参数作为参数。

请注意，编译器应该优化赋值以避免复制 - 即它将通过将create + assign + copy优化为create创建一个Rectangle（rect）。这只适用于从函数返回时创建新对象。

Note that when you return by value as in the code below, the compiler should optimize the assignment to avoid the copy - i.e. it will just create a single Rectangle (rect) by optimizing the create+assign+copy into a create. This only works when you create the new object when returning from the function.

Rectangle GetRect( void ) const
{
    return Rectangle( x, y, w, h );
}

Rectangle rect = theBox.GetRect();

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