std::string formatting like sprintf

Question

I have to format std::string with sprintf and send it into file stream. How can I do this?

Answer 1

In order to format std::string in a 'sprintf' manner, call snprintf (arguments nullptr and 0) to get length of buffer needed. Write your function using C++11 variadic template like this:

#include 
#include 
#include 

template< typename... Args >
std::string string_sprintf( const char* format, Args... args ) {
  int length = std::snprintf( nullptr, 0, format, args... );
  assert( length >= 0 );

  char* buf = new char[length + 1];
  std::snprintf( buf, length + 1, format, args... );

  std::string str( buf );
  delete[] buf;
  return str;
}

Compile with C++11 support, for example in GCC: g++ -std=c++11

Usage:

  std::cout << string_sprintf("%g, %g\n", 1.23, 0.001);