C++: how to get fprintf results as a std::string w/o sprintf

I am working with an open-source UNIX tool that is implemented in C++, and I need to change some code to get it to do what I want. I would like to make the smallest possible change in hopes of getting my patch accepted upstream. Solutions that are implementable in standard C++ and do not create more external dependencies are preferred.

Here is my problem. I have a C++ class -- let's call it "A" -- that currently uses fprintf() to print its heavily formatted data structures to a file pointer. In its print function, it also recursively calls the identically defined print functions of several member classes ("B" is an example). There is another class C that has a member std::string "foo" that needs to be set to the print() results of an instance of A. Think of it as a to_str() member function for A.

In pseudocode:

class A {
public:
  ...

  void print(FILE* f);
  B b;

  ...  
};

...

void A::print(FILE *f)
{
  std::string s = "stuff";
  fprintf(f, "some %s", s);
  b.print(f);
}

class C {
  ...
  std::string foo;
  bool set_foo(std::str);
  ...
}

...

A a = new A();
C c = new C();

...

// wish i knew how to write A's to_str()
c.set_foo(a.to_str());

I should mention that C is fairly stable, but A and B (and the rest of A's dependents) are in a state of flux, so the less code changes necessary the better. The current print(FILE* F) interface also needs to be preserved. I have considered several approaches to implementing A::to_str(), each with advantages and disadvantages:

1. Change the calls to fprintf() to sprintf()

   • I wouldn't have to rewrite any format strings
   • print() could be reimplemented as: fprint(f, this.to_str());
   • But I would need to manually allocate char[]s, merge a lot of c strings , and finally convert the character array to a std::string

2. Try to catch the results of a.print() in a string stream

   • I would have to convert al