const引用临时对象的成员 [英] const reference to member of temporary object

问题描述

这是已知C ++的特性，const引用扩展从函数返回的临时对象的生命周期，但是使用常量引用从函数返回的临时对象的成员也可以接受？

It is known feature of C++ that a const reference extends the life time of the temporary object returned from a function, but is it acceptable to use constant reference to the member of the temporary object returned from the function?

示例：

#include <string>

std::pair<std::string, int> getPair(int n)
{
    return {std::to_string(n), n};
}

int main(int, char*[])
{
    const int x = 123456;
    const auto& str = getPair(x).first;
    printf("%d = %s\n", x, str.c_str());    
    return 0;
}

输出：

123456 = 123456

推荐答案

是的，这个代码是完全可以接受的。规则，根据标准是（[class.temporary]）：

Yes, this code is perfectly acceptable. The rules, according to the standard are ([class.temporary]):

1. 有两个上下文，其中临界被破坏在一个
   不同的点，而不是结束。第一个上下文
   是当调用默认构造函数来初始化
   数组的元素时。如果构造函数有一个或多个默认参数，那么在构造下一个数组元素（如果有的话）之前，在默认参数中创建的每个临时的
   破坏是
   。

1. There are two contexts in which temporaries are destroyed at a different point than the end of the fullexpression. The first context is when a default constructor is called to initialize an element of an array. If the constructor has one or more default arguments, the destruction of every temporary created in a default argument is sequenced before the construction of the next array element, if any.

第二个上下文是引用绑定到临时文件。引用绑定到的
临时绑定的引用绑定的子对象的
完整对象的临时
在参考...

The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference...

正如您所看到的，

请注意，第一个 $ c>符合子对象[intro.object]：

Note that first does qualify as a subobject [intro.object]:

1. 包含其他对象，称为子对象。一个子对象可以
   是一个成员子对象（9.2），一个基类子对象（子句10）或一个
   数组元素。不是任何其他对象
   的子对象的对象称为完整对象。

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