c++ - 对临时对象成员的 const 引用

Question

常量引用延长函数返回的临时对象的生命周期是C++的known feature，但是对函数返回的临时对象的成员使用常量引用是否可以接受？

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

Answer 1

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

4. 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.

5. 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...

如您所见，突出显示的行清楚地表明对子对象的绑定(bind)引用是可以接受的，因为完整的对象也必须延长其生命周期。

请注意，first 确实有资格作为子对象 [intro.object]:

2. Objects can contain other objects, called subobjects. A subobject can be a member subobject (9.2), a base class subobject (Clause 10), or an array element. An object that is not a subobject of any other object is called a complete object.