C++编译器的返回值优化

原创已于 2024-02-11 21:32:49 修改 · 535 阅读

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#开发语言 #c++

于 2024-02-11 21:29:51 首次发布

文章讨论了C++编译器如何通过返回值优化(RVO)机制，避免对临时对象进行不必要的拷贝或移动操作，即使有移动构造函数，编译器也会直接构造临时对象到目标对象的内存中。

C++编译器的返回值优化

C++编译器的返回值优化

C++编译器的返回值优化

在对右值概念的学习验证中发现函数构造的实例返回后不会被销毁（尽管已经跳出作用域），而是直接被使用

上代码

#include <iostream>
#include <cstdio>
#include <cassert>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <random>
#include <utility>
#include <string>

using namespace std;

class MoveOrCopy {
private:
    int a;
public:
    MoveOrCopy() : a(0) {
        printf("default consult: %p\n", this);
    }

    MoveOrCopy(const MoveOrCopy &m) : a(m.a) {
        printf("copy consult is called: %p\n", this);
    }

    MoveOrCopy(MoveOrCopy &&m) noexcept: a(m.a) {
        printf("move consult is called: %p\n", this);
    }

    virtual ~MoveOrCopy() {
        printf("MoveOrCopy %p:%d is deleted\n", this, this->a);
    }

    MoveOrCopy &operator=(const MoveOrCopy &m) {
        this->a = m.a;
        printf("copy valued is called\n");
    }
};

MoveOrCopy create() {
    return MoveOrCopy();
}

MoveOrCopy createWithLocal() {
    MoveOrCopy t = MoveOrCopy();
    return t;
}

MoveOrCopy createCondition(bool b = false) {
    return b ? MoveOrCopy() : MoveOrCopy();
}

int main() {
    printf("d:\n");
    MoveOrCopy d = create();
    printf("e:\n");
    MoveOrCopy e = createWithLocal();
    printf("f:\n");
    MoveOrCopy f = createCondition();
    return 0;
}

运行结果

d:
default consult: 000000D4B12FFB48
e:
default consult: 000000D4B12FFAD8
move consult is called: 000000D4B12FFB38
MoveOrCopy 000000D4B12FFAD8:0 is deleted
f:
default consult: 000000D4B12FFAC8
move consult is called: 000000D4B12FFB28
MoveOrCopy 000000D4B12FFAC8:0 is deleted
MoveOrCopy 000000D4B12FFB28:0 is deleted
MoveOrCopy 000000D4B12FFB38:0 is deleted
MoveOrCopy 000000D4B12FFB48:0 is deleted