为什么不在这里调用复制构造函数？ [英] Why copy constructor is not called here?

问题描述

以下代码：

    #include<iostream>
    using namespace std;

    class Test
    {
    public:
       Test(const Test &t) { cout<<"Copy constructor called"<<endl;}
       Test()        { cout<<"Constructor called"<<endl;}
    };

    Test fun()
    {
        cout << "fun() Called\n";
        Test t;
        return t;
    }

    int main()
    {
        Test t1;
        Test t2 = fun();
        return 0;
    }

我真的很困惑，当调用复制构造函数时？喜欢如果我运行上面的程序，拷贝构造函数不会被调用。这意味着如果我搞砸了参数传递给复制构造函数（消除const关键字），它不应该显示任何编译器错误。但它显示

I am really confused regarding when the copy constructor is called ? Like if I am running the above program copy constructor is not called. That means if I am messing up with the parameters passed to the copy constructor (eliminating const keyword) it shouldn't show any compiler error. But its showing

没有匹配的函数调用'Test :: Test（Test）'

"no matching function for call to 'Test::Test(Test)' "

此外，fun（）返回一个类型为test的对象，它是在fun（）执行期间创建的。为什么不在这里调用复制构造函数？

Moreover, the fun() is returning an object of type test , which is created during fun() execution. Why the copy constructor is not called here ?

    int main()
    {
        fun();
        return 0;
    }

同样，如果我对主函数进行以下更改为什么仅调用复制构造函数一次，不是两次？

also if I am making following changes to main function why copy constructor is called only once , not twice ?

    int main()
    {
        Test t2 = fun();
        Test t3 = t2;
        return 0;
    }

推荐答案

此处使用的不是复制构造函数，因为您的编译器中启用了 NRVO 。您应该指定

It is because copy initialization is used here, not copy constructor, due to the NRVO enabled in your compiler. You should specify

-fno-elide-constructors

在gcc上标记

C ++标准允许实现省略创建一个临时的
，它只用于初始化同一类型的另一个对象。
指定此选项禁用该优化，并强制G ++到
在所有情况下调用复制构造函数。

The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

或者在VS（/ O1和/ O2 cl / Od program.cpp library / 8f8h5cxt.aspxrel =nofollow>启用NRVO ）

or compile it with cl /Od program.cpp on VS (/O1 and /O2 enables NRVO)

C ++：避免​​使用return语句复制

这篇关于为什么不在这里调用复制构造函数？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！