为什么此函数的一次调用使用 NRVO，而同一函数的另一次调用则不使用 NRVO

Question

vector

是一个具有两个公共成员的结构体；一个打印

integer constructor called

的参数化构造函数；打印

copy constructor called

的复制构造函数；打印

move constructor called

.

的移动构造函数

如果

makeVectorx5y5IfNotZero

是

{5, 5}

，则函数

arg

返回向量

0

。否则返回

{2, 2}

。

main

函数使用

vector A

构造

makeVectorx5y5IfNotZero(1)

并使用

vector B

构造

makeVectorx5y5IfNotZero(0)

。这是代码

#include <iostream>

struct vector{
    int X, Y;
    vector(int X, int Y) : X(X), Y(Y){
        printf("integer constructor called\n");
    }
    vector(const vector& other): X(other.X), Y(other.Y){
        printf("copy constructor called\n");
    }
    vector(vector&& other): X(other.X), Y(other.Y){
        printf("move constructor called\n");
    }
};

vector makeVectorx5y5IfNotZero(bool Arg){
    if(Arg){
        vector A(0, 0);
        A.X = 5;
        A.Y = 5;
        return A;
    }
    else {
        vector B(1,1);
        B.X = 2;
        B.Y = 2;
        return B;
    }
}

int main(){
    printf("Making Vector\n");
    vector A = makeVectorx5y5IfNotZero(1);
    printf("Made Vector\n");
    printf("Making Another Vector\n");
    vector B = makeVectorx5y5IfNotZero(0);
    printf("Made Vector\n");
}

这段代码的输出是：

C:\copy elision>g++ --version
g++ (Rev3, Built by MSYS2 project) 14.1.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

C:\copy elision> g++ question.cpp -std=c++23

C:\copy elision>a
Making Vector
integer constructor called
Made Vector
Making Another Vector
integer constructor called
move constructor called
Made Vector

C:\copy elision> 

但是，由于 NRVO，我期望以下输出

Making Vector
integer constructor called
Made Vector
Making Another Vector
integer constructor called
Made Vector

我将非常感谢您解释为什么'makeVectorx5y5IfNotZero

B

B

is moved into the local variable

main`的局部变量

of

而不是仅仅在同一内存地址构造单个变量。

Answer 1

正如 @PepijnKramer 所说，这是 gcc 的限制。在此视频中对此进行了更详细的讨论 C++ Weekly - Ep 456 - RVO + Trivial Types = Faster Code