std::string::c_str() 结果的生命周期是多少?

2021-12-05 00:00:00 string c++ lifetime c-strings c-str

在我的一个程序中，我必须与一些使用 const char* 的遗留代码交互.

In one of my programs, I have to interface with some legacy code that works with const char*.

假设我有一个如下所示的结构:

Lets say I have a structure which looks like:

struct Foo
{
  const char* server;
  const char* name;
};

我的上层应用只处理std::string，所以想到了用std::string::c_str()来取回const char* 指针.

My higher-level application only deals with std::string, so I thought of using std::string::c_str() to get back const char* pointers.

但是 c_str() 的生命周期是多少?

But what is the lifetime of c_str() ?

我可以在不面对未定义行为的情况下做这样的事情吗?

Can I do something like this without facing undefined behavior ?

{
  std::string server = "my_server";
  std::string name = "my_name";

  Foo foo;
  foo.server = server.c_str();
  foo.name = name.c_str();

  // We use foo
  use_foo(foo);

  // Foo is about to be destroyed, before name and server
}

还是我应该立即将 c_str() 的结果复制到另一个地方?

Or am I supposed to immediately copy the result of c_str() to another place ?

谢谢.

推荐答案

如果 std::string 被销毁或非-const 字符串的成员函数被调用.因此，如果您需要保留它，通常您会想要制作它的副本.

The c_str() result becomes invalid if the std::string is destroyed or if a non-const member function of the string is called. So, usually you will want to make a copy of it if you need to keep it around.

在您的示例中，c_str() 的结果似乎是安全使用的，因为在该范围内不会修改字符串.(但是，我们不知道 use_foo() 或 ~Foo() 可能对这些值做什么；如果他们将字符串复制到别处，那么他们应该做一个真正的复制，而不仅仅是复制char指针.)

In the case of your example, it appears that the results of c_str() are used safely, because the strings are not modified while in that scope. (However, we don't know what use_foo() or ~Foo() might be doing with those values; if they copy the strings elsewhere, then they should do a true copy, and not just copy the char pointers.)

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