How do I perform string formatting to a static buffer in C++?

I am working in a section of code with very high performance requirements. I need to perform some formatted string operations, but I am trying to avoid memory allocations, even internal library ones.

In the past, I would have done something similar to the following (assuming C++11):

constexpr int BUFFER_SIZE = 200;
char buffer[BUFFER_SIZE];
int index = 0;
index += snprintf(&buffer[index], BUFFER_SIZE-index, "Part A: %d\n", intA);
index += snprintf(&buffer[index], BUFFER_SIZE-index, "Part B: %d\n", intB);
// etc.

I would prefer to use all C++ methods, such as ostringstream, to do this instead of the old C functions.

I realize I could use std::string::reserve and std::ostringstream to procure space ahead of time, but that will still perform at least one allocation.

Does anyone have any suggestions?

Thanks ahead of time.

My thanks to all that posted suggestions (even in the comments).

I appreciate the suggestion by SJHowe, being the briefest solution to the problem, but one of the things I am looking to do with this attempt is to start coding for the C++ of the future, and not use anything deprecated.

The solution I decided to go with stems from the comment by Remy Lebeau:

#include <iostream>  // For std::ostream and std::streambuf
#include <cstring>   // For std::memset

template <int bufferSize>
class FixedBuffer : public std::streambuf
{
public:
   FixedBuffer()
      : std::streambuf()
   {
      std::memset(buffer, 0, sizeof(buffer));
      setp(buffer, &buffer[bufferSize-1]);         // Remember the -1 to preserve the terminator.
      setg(buffer, buffer, &buffer[bufferSize-1]); // Technically not necessary for an std::ostream.
   }

   std::string get() const
   {
      return buffer;
   }

private:
   char buffer[bufferSize];
};

//...

constexpr int BUFFER_SIZE = 200;
FixedBuffer<BUFFER_SIZE> buffer;
std::ostream ostr(&buffer);

ostr << "PartA: " << intA << std::endl << "PartB: " << intB << std::endl << std::ends;