返回字符串及其 .c_str() 的生命周期 [英] Lifetime of returned strings and their .c_str()

问题描述

我遇到过这种模式的多个实例(boost::filesystem 仅用作示例):

I've come across multiple instance of this pattern (with boost::filesystem only used as example):

boost::filesystem::path path = ...;
someFunctionTakingCStrings(path.string().c_str());

哪里

const std::string path::string() const
{
  std::string tmp = ...
  return tmp;
}

虽然我从来没有遇到过这种模式的问题，但我想知道sting()返回的字符串什么时候被销毁，访问c_str()的代码是否是安全，因为 c_str() 生命周期绑定到 std::string 生命周期.>

Although I have never experienced problem with this pattern, I was wondering when the string returned by sting() is destroyed and whether the code accessing the c_str() is safe as the c_str() lifetime is bound to std::string lifetime.

推荐答案

someFunctionTakingCStrings(path.string().c_str()); 是安全的，因为标准保证匿名临时的生命周期path.string() 在函数调用中存活下来.所以c_str()返回的指针是someFunctionTakingCStrings的有效参数.

someFunctionTakingCStrings(path.string().c_str()); is safe since the standard guarantees that the lifetime of the anonymous temporary path.string() survives the function call. So the pointer returned by c_str() is a valid parameter for someFunctionTakingCStrings.

const std::string path::string() const 是安全的，因为从概念上讲，您返回的是 tmp 的值副本，尽管实际上编译器会优化值副本(称为命名返回值优化的过程).

const std::string path::string() const is safe since, conceptually, you are returning a value copy of tmp, although in practice a compiler will optimise out the value copy (a process called named return value optimisation).

类似于 const std::string&path::string() const 与您拥有的函数体相同的函数体将不会被定义(因为引用会悬垂)，并且

Something like const std::string& path::string() const with the same function body as the one you have would not be defined (since the reference would dangle), and

const char* ub_server()
{
    std::string s = "Hello";
    return s.c_str();
}

也是未定义的，因为 s 在函数返回时超出范围.

is also undefined, as s is out of scope by the time the function returns.

最后，请注意，在标准 C++ 中不允许允许将指向匿名临时对象的指针作为函数调用中的参数，尽管令人讨厌的是，Visual C++ 允许将其作为扩展.

Finally, note that taking a pointer to an anonymous temporary as a parameter in a function call is not allowed in standard C++ although annoyingly, Visual C++ allows it as an extension.

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