从函数返回值是如何工作的?

2022-01-22 00:00:00 stack c++

我最近遇到了一个严重的错误，我忘记在函数中返回一个值.问题是，即使没有返回任何内容，它在 Linux/Windows 下也能正常工作，并且只在 Mac 下崩溃.当我打开所有编译器警告时，我发现了这个错误.

I recently had a serious bug, where I forgot to return a value in a function. The problem was that even though nothing was returned it worked fine under Linux/Windows and only crashed under Mac. I discovered the bug when I turned on all compiler warnings.

所以这里是一个简单的例子:

So here is a simple example:

#include <iostream>

class A{
public:
    A(int p1, int p2, int p3): v1(p1), v2(p2), v3(p3)
    {
    }

    int v1;
    int v2;
    int v3;
};

A* getA(){
    A* p = new A(1,2,3);
//  return p;
}

int main(){

    A* a = getA();

    std::cerr << "A: v1=" << a->v1 << " v2=" << a->v2 << " v3=" << a->v3 << std::endl;  

    return 0;
}

我的问题是如何在 Linux/Windows 下运行而不会崩溃?底层的值返回是怎么做的?

My question is how can this work under Linux/Windows without crashing? How is the returning of values done on lower level?

推荐答案

在 Intel 架构上，简单的值(整数和指针)通常在 eax 寄存器中返回.该寄存器(除其他外)还用作在内存中移动值时的临时存储和计算期间的操作数.因此，该寄存器中剩余的任何值都被视为返回值，在您的情况下，结果正是您想要返回的值.

On Intel architecture, simple values (integers and pointers) are usually returned in eax register. This register (among others) is also used as temporary storage when moving values in memory and as operand during calculations. So whatever value left in that register is treated as the return value, and in your case it turned out to be exactly what you wanted to be returned.

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