Returning c_str from a function

Question

This is from a small library that I found online:

 const char* GetHandStateBrief(const PostFlopState* state)
{
    static std::ostringstream out;

... rest of the function ...

 return out.str().c_str()

Now in my code I am doing this:

const char *d = GetHandStateBrief(&post);
std::cout<< d << std::endl;

Now, at first d contained garbage. I then realized that the c string I am getting from the function is destroyed when the function returns because std::ostringstream is allocated on the stack. So I added:

return strdup( out.str().c_str());

And now I can get the text I need from the function.

I have two questions:

1) Am I understanding this correctly?

2) I later noticed that the ostringstream was was allocated with static storage. Doesn't that mean that the object is supposed to stay in memory until the program terminates? and if so , then why can't I access the string?

Answer 1

strdup allocates a copy of the string on the heap, which you have to free manually later (with free() I think). If you have the option, it would be much better to return std::string.

The static storage of out doesn't help, because .str() returns a temporary std::string, which is destroyed when the function exits.

Answer 2

In GetHandStateBrief, variable out does not need to be static. You need an explicit static string to replace the temporary that was being created in your original call to out.str():

static std::string outStr;
std::ostringstream out;
... rest of function ...
outStr = out.str();
return outStr.c_str();

Answer 3

strdup() returns a char* pointer that is pointing to memory on the heap. You need to free() it when you're done with it, but yes, that will work.

The static local variable std::ostringstream out makes no sense in this case, unless the std::string being returned was also static which your observation is showing to be not true.

Answer 4

You're right that out is a static variable allocated on the data segment. But out.str() is a temporary allocated on the stack. So when you do return out.str().c_str() you're returning a pointer to a stack temporary's internal data. Note that even if a string is not a stack variable, c_str is "only granted to remain unchanged until the next call to a non-constant member function of the string object."

I think you've hit on a reasonable workaround, assuming you can't just return a string.