引用未命名的临时对象（life time） [英] Reference to an unnamed temporary object (life time)

问题描述

在阅读此回答之后，ildjarn ，我写了以下示例，它看起来像一个未命名的临时对象具有与其参考相同的生命时间！

After reading this answer from ildjarn, I wrote the following example, and it looks like an unnamed temporary object has the same life time as its reference!

• 这是怎么可能的？


• 是否在C ++标准中指定？


• 哪个版本？

源代码： >

Source code:

#include <iostream>  //cout
#include <sstream>   //ostringstream 

int main ()
{
        std::ostringstream oss;
        oss << 1234;

        std::string const& str = oss.str();
        char        const* ptr = str.c_str();

        // Change the stream content
        oss << "_more_stuff_";
        oss.str(""); //reset
        oss << "Beginning";
        std::cout << oss.str() <<'\n';

        // Fill the call stack
        // ... create many local variables, call functions...

        // Change again the stream content
        oss << "Again";
        oss.str(""); //reset
        oss << "Next should be '1234': ";
        std::cout << oss.str() <<'\n';

        // Check if the ptr is still unchanged
        std::cout << ptr << std::endl;
}

执行：

> g++ --version
g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
Copyright (C) 2006 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.
> g++ main.cpp -O3
> ./a.out
Beginning
Next should be '1234':
1234

推荐答案

如何实现？

How come this is possible?

因为标准说，所以，因为它被认为是有用的。 rvalue引用和 const lvalue引用扩展临时表的生命周期：

Because the standard says so, because it's deemed useful. rvalue references and const lvalue references extend the lifetime of temporaries:

[C ++ 11：12.2 / 5]： [..] 引用绑定的临时或临时，子引用绑定的子对象将在引用的生命周期中保持
，除了 [..]

[C++11: 12.2/5]: [..] 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, except [..]

$ b $在 [C ++ 11：8.5.3 / 5] 中要求我们不将临时绑定到非 - const lvalue references。

and exhaustive wording in [C++11: 8.5.3/5] requires that we shall not bind temporaries to non-const lvalue references.

？
哪个版本？

Is it specified in the C++ standard? Which version?

是的。所有人。

这篇关于引用未命名的临时对象（life time）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！