c++ - 我是否正确测试了返回值优化？

Question

考虑:

struct Measures {
    char name[128]; // added in response to comments
    int   age;
    float weight;
};

Measures foo() {
    Measures res;
    cout << &res << endl;
    return res;
}

int main() {
    Measures m = foo();
    cout << &m << endl;
    system("pause");
    return 0;
}

在 Visual Studio 中运行此代码得到的输出是:

004FFA30
004FFB28

根据两个地址不同，我是否正确地得出没有执行返回值优化的结论？如果是这样，为什么在这种情况下不执行它(我发现了几篇涉及 Visual Studio 2017(版本 15.9.14)不执行返回值优化的帖子，但它们似乎都不是关于这种特殊情况的)？

Answer 1

https://godbolt.org/z/FCRtY9

正如评论中所暗示的，这与结构的大小有关。如果该结构适合寄存器，则地址可能会更改。

创建更大的结构后，地址保持不变。

请注意，复制省略一定会发生:https://stackoverflow.com/a/48881336/461597

#include <iostream>
using std::cout;
using std::endl;

struct Measures {
    int   age;
    float weight;
};

struct BigMeasures {
    Measures m;
    int foo, bar, moo, mar;
};

Measures foo() {
    Measures res;
    cout << &res << endl;
    return res;
}

BigMeasures bar() {
    BigMeasures res;
    std::cout << &res << "\n";
    return res;
}

int main() {
    std::cout << "Small Measures:\n";
    Measures m = foo();
    cout << &m << endl;
    std::cout << "Big Measures:\n";
    BigMeasures bm = bar();
    std::cout << &bm << "\n";
    return 0;
}