basic_ostringstream :: str悬空指针 [英] basic_ostringstream::str dangling pointer

问题描述

最近我在以下代码中发现了该错误：

Recently I found the bug in the following code:

ostringstream o;
o << "some string";
const char* s = o.str().c_str(); // empty string instead of expected "some string"

cppreference.com对此进行了解释： str返回的基础字符串的副本是一个临时对象，该对象将在表达式的末尾被破坏，因此直接在str（）的结果上调用c_str（）（例如，在auto * ptr = out.str（ ）.c_str（）;）导致指针悬空。

This is explained by cppreference.com: the copy of the underlying string returned by str is a temporary object that will be destructed at the end of the expression, so directly calling c_str() on the result of str() (for example in auto *ptr = out.str().c_str();) results in a dangling pointer.

我没有任何问题可以解决此错误，但是，我在其中有很多地方项目，如下所示：

I don't have any problem fixing this bug, however, I have many places in the projects, which look like this:

ostringstream o;
o << "error description";
throw my_exception(o.str().c_str());

...

my_exception::my_exception(const char* s) :
    message(s)     // message is std::string
{}

此代码是否具有未定义的行为，例如第一个代码片段？

Does this code has undefined behavior, like the first code fragment?

推荐答案

否，消息是std :: string，因此您需要在此时获取char缓冲区的内容的副本。

No, message(s) is a std::string so you take a copy of the contents of the char buffer at that point.

临时函数将持续使用您调用的函数的范围-在这种情况下为构造函数。

The temporary lasts for the scope of the function you call it with - in this case the constructor.

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