sprintf returning std::string

main.cpp

#include <cstdio>
#include <cstdarg>
#include <string>
#include <exception>

// #include <cstdlib>
// #include <vector>
// #include <iostream>


std::string string_printf(const char* const format, ...)
{

  va_list arg_ptr;
  va_start(arg_ptr, format);
  const int ret_prn_pre = vsnprintf(NULL, 0, format, arg_ptr);
  va_end(arg_ptr);

  // fprintf(stderr, "[%s] ret_prn_pre=%d\n", __func__, ret_prn_pre);

  if(ret_prn_pre < 0) {
    throw std::invalid_argument("format is invalid");
  }

  const size_t size = ret_prn_pre+1;
  std::string result_string;
  result_string.resize(size);

  va_start(arg_ptr, format);
  const int ret_prn = vsnprintf(&result_string[0], size, format, arg_ptr);
  va_end(arg_ptr);

  // fprintf(stderr, "[%s] ret_prn=%d\n", __func__, ret_prn);

  if(ret_prn >= size) {
    throw std::length_error("size of buffer is smaller than needed.");
  }

  return result_string;

};


template<typename T>
std::string to_string(const T);

template<>
std::string to_string<int>(const int i)
{ return string_printf("%d", i); }

template<>
std::string to_string<long int>(const long int i)
{ return string_printf("%ld", i); }

template<>
std::string to_string<long long int>(const long long int i)
{ return string_printf("%lld", i); }


template<>
std::string to_string<unsigned int>(const unsigned int i)
{ return string_printf("%u", i); }

template<>
std::string to_string<unsigned long int>(const unsigned long int i)
{ return string_printf("%lu", i); }

template<>
std::string to_string<unsigned long long int>(const unsigned long long int i)
{ return string_printf("%llu", i); }


// template<>
// std::string to_string<size_t>(const size_t i)
// { return string_printf("%zu", i); }

// template<>
// std::string to_string<ssize_t>(const ssize_t i)
// { return string_printf("%zd", i); }


template<>
std::string to_string<float>(const float f)
{ return string_printf("%g", f); }

template<>
std::string to_string<double>(const double f)
{ return string_printf("%g", f); }

template<>
std::string to_string<long double>(const long double f)
{ return string_printf("%Lg", f); }


template<>
std::string to_string<char*>(char* const ptr)
{ return ptr; }

template<>
std::string to_string<void*>(void* const ptr)
{ return string_printf("%p", ptr); }

// template<typename T>
// std::string to_string<T*>(T* ptr)
// { return string_printf("%p", ptr); }


#define PRINTVAR(var) fprintf(stderr, #var "=%s\n" , to_string(var).c_str())


struct {
  double a;
  ssize_t s;
} o;

int main()
{

  o.a = 1.2345;
  o.s = 1234;

  //const std::string str = string_printf("test %d %.18g", 1, 1.23456789);
  //printf("str=\"%s\"\n", to_string((void*)&s).c_str());

  PRINTVAR(o.a);
  PRINTVAR(o.s);


  return 0;
}