stringstream :: str复制生命周期 [英] stringstream::str copy lifetime

问题描述

第一次使用 stringstream 时，许多C ++程序员错过了（读取：me）是由 stringstream :: str（）是一个临时的，它持续到它使用的表达式的结尾。但是，我不明白：

Something a lot of C++ programmers miss (read: me) when first using stringstreams is the fact that the copy returned by stringstream::str() is a temporary, which lasts until the end of the expression it's used in. However, I don't understand:

1. 如何做。查看libstdc ++的 sstream 头文件，我只看到一个副本被创建和返回。


2. 为什么这是期望的行为，特别是因为它是一个常见的秘密。如果正在发送副本，为什么我不能拥有该副本？

1. How this is done. Looking in the sstream header for libstdc++, I only see a copy being made and returned. How is the lifetime restricted?
2. Why this is the desired behavior, especially since it is such a common gotcha. If a copy is being made anyway, why can't I take ownership of it?

请注意，这不是< a href =http://stackoverflow.com/questions/1374468/c-stringstream-string-and-char-conversion-confusion> C ++ stringstream，string和char *转换混淆。这解释了行为和解决方法;我正在寻找机制和理由。

Note that this is not a duplicate of C++ stringstream, string, and char* conversion confusion. That goes over explaining the behavior and workarounds; I'm looking for mechanics and rationale.

推荐答案

这不是 stringstream 。这是 c_str 函数的问题 - 它返回指向 char * 表示的指针 std :: string ，但它只在原始字符串的生命周期内存在。当你调用 char * str = ss.str（）。c_str（）时，会发生以下情况：

This is not a problem of stringstream. It is the problem of c_str function - it returns pointer to a char* representation of std::string, but it lives only during lifetime of an original string. When you call char *str = ss.str().c_str() the following actually happens:

string tmp = ss.str();
char *str = tmp.c_str();
tmp.~string (); // after that line the pointer `str` is no longer valid

c_str 是一个危险的功能，仅为兼容性和速度的目的，您应避免使用它。

c_str is a dangerous function provided only for compatibility and speed purposes and you should avoid using it.

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