编译器关于销毁临时文件的策略
我一直在玩下面的代码。 file_string 返回一个临时字符串，该字符串只能“存活”到语句结束。在 Visual Studio 2008 中，当您使用 pTempFolder 时，它会按预期包含垃圾。但在 Linux 中，使用 Intel 编译器 11.0，pTempFolder 仍然指向有效的字符串。编译器对于临时对象的销毁是否有不同的策略，即急切的（Visual）与惰性的（Intel）？或许这只是一个巧合？
boost::filesystem wpathTempFolder("/tmp");
const wchar_t* const pTempFolder = wpathTempFolder.file_string().c_str();
// use pTempFolder
顺便说一句，这是 boost 文件系统版本 2。我还看到 file_string 在 boost 文件系统版本 3 中已被弃用。并且有一个新的 c_str 方法可以在字符串&，而不是临时字符串。
/*filesystem 2*/
const string_type file_string() const;
/*filesystem 3*/
const string_type& native() const; // native format, encoding
const value_type* c_str() const; // native().c_str()
I've been playing with the following piece of code. file_string returns a temporary string that should only "live" until the end of the statement. In Visual Studio 2008, when you use pTempFolder, it contains rubbish as expected. In Linux though, with Intel compiler 11.0, pTempFolder still points to a valid string. Do compilers have different policies regarding the destruction of temporaries, kind of eager (Visual) versus lazy (Intel)? Or maybe this is just a coincidence?
boost::filesystem wpathTempFolder("/tmp");
const wchar_t* const pTempFolder = wpathTempFolder.file_string().c_str();
// use pTempFolder
BTW, that is boost filesystem version 2. I've also seen that file_string is being deprecated in boost filesystem version 3. And that there is a new c_str method that operates over a string&, instead of over a temporary string.
/*filesystem 2*/
const string_type file_string() const;
/*filesystem 3*/
const string_type& native() const; // native format, encoding
const value_type* c_str() const; // native().c_str()
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很可能，该字符串仍然无效，只是碰巧该内存部分尚未在操作系统级别取消分配，并且它“碰巧”可以工作。该程序表现出未定义的行为，其中始终包括“可能会继续工作，就好像没有发生任何问题一样”。 Visual Studio 在这里完全正确地使您的程序或几乎任何东西崩溃。
Likely, the string is still invalid, it just so happens that that section of memory hasn't yet been de-allocated at the operating system level and it "happens" to work. This program exhibits undefined behaviour- which always includes "may continue to work as if nothing went wrong". Visual Studio is completely correct here to crash your program or pretty much anything.
临时对象的生命周期（有少数例外）直到
完整表达式结束。指向的数组对象的生命周期
通过
std::string::c_str()的返回值不超过字符串对象本身的长度（并且可能更短，如果有的话）
在字符串上调用非常量函数）。访问
对象在其生命周期结束后是未定义的行为，所以
您无法从编译器的行为中得出任何结论。
The lifetime of a temporary (with a few exceptions) is until the
end of the full expression. The lifetime of the array object pointed to
by the return value of
std::string::c_str()does not excedethat of the string object itself (and may be shorter, if any
non-const functions are called on the string). Accessing an
object after its lifetime has ended is undefined behavior, so
you cannot draw any conclusions from what the compiler does.