What is the lifetime of the result of std::string::c_str()?

Question

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

Lets say I have a structure which looks like:

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

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

But what is the lifetime of c_str() ?

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
}

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

Thank you.

Solution 1

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.

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.)

Solution 2

Technically your code is fine.

BUT you have written in such a way that makes it easy to break for somebody that does not know the code. For c_str() the only safe usage is when you pass it as a parameter to a function. Otherwise you open yourself up-to maintenance problems.

Example 1:

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

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

  //
  // Imagine this is a long function
  // Now a maintainer can easily come along and see name and server
  // and would never expect that these values need to be maintained as
  // const values so why not re-use them

  name += "Martin";
  // Oops now its broken.

  // We use foo
  use_foo(foo);

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

So for maintenance make it obvious:

Better solution:

{
  // Now they can't be changed.
  std::string const server = "my_server";
  std::string const name   = "my_name";

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

  use_foo(foo);    
}

But if you have const strings you don't actually need them:

{
  char const* server = "my_server";
  char const* name   = "my_name";

  Foo foo;
  foo.server = server;
  foo.name   = name;

  use_foo(foo);
}

OK. For some reason you want them as strings:
Why not use them only in the call:

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

  // guaranteed not to be modified now!!!     
  use_foo(Foo(server.c_str(), name.c_str());
}

Solution 3

It is valid until one of the following happens to the corresponding string object:

• the object is destroyed
• the object is modified

You're fine with your code unless you modify those string objects after c_str()s are copied into foo but before use_foo() is called.

Solution 4

Return value of c_str() is valid only until the next call of a nonconstant member function for the same string

Solution 5

The const char* returned from c_str() is only valid until the next non-const call to the std::string object. In this case you're fine because your std::string is still in scope for the lifetime of Foo and you aren't doing any other operations that would change the string while using foo.

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Comments

[SomethingSomething]

Happened to me when I defined a local string in a function and returned .c_str(). I didn't understand why sometimes I get only parts of the string, until I understood that the const char* does not live forever, but until the string is destroyed

[GuruM]

c_str() pointer could be invalid if the std::string object is a automatic object going out of scope or in call to a thread creating function.

[Mathew Kurian]

Can you please explain non-const member function of the string is called.?

[kevinthompson]

A "non-const member function" is any member function that is not marked with the const keyword. Such a function might mutate the string's contents, in which case the string might need to reallocate the memory for the null-terminated version of the string returned by c_str(). For example, size() and length() are const, so you can call them without worrying about the string changing, but clear() is not const.