Question

Possible Duplicate:
Convert string to int C++
What’s the fastest way to check if a string is a number?

I need to convert string to int and I need to know if converting was successful. But number can be zero, so I can’t use atoi and others. String to convert I read from file.

Answer 1

you can use this template function, but this is not just for converting a string to an int – it is to convert a string to every type:

template <typename T>
T ConvertString( const std::string &data )
{
  if( !data.empty( ))
  {
    T ret;
    std::istringstream iss( data );
    if( data.find( "0x" ) != std::string::npos )
    {
      iss >> std::hex >> ret;
    }
    else
    {
      iss >> std::dec >> ret;
    }

    if( iss.fail( ))
    {
      std::cout << "Convert error: cannot convert string '" << data << "' to value" << std::endl;
      return T( );
    }
    return ret;
  }
  return T( );
}

If you want to now if the convert was successfull, then return a specific value in the if iss.fail() or pass a second reference argument to the function and set the value to false if it failed.

You can use it like this:

uint16_t my_int = ConvertString<uint16_t>("15");

If you like the solution with the reference argument, here an example:

#include <iostream>
#include <sstream>
#include <string>

#include <inttypes.h>

template <typename T>
T ConvertString(const std::string &data, bool &success)
{
  success = true;
  if(!data.empty())
  {
    T ret;
    std::istringstream iss(data);
    if(data.find("0x") != std::string::npos)
    {
      iss >> std::hex >> ret;
    }
    else
    {
      iss >> std::dec >> ret;
    }

    if(iss.fail())
    {
      success = false;
      return T();
    }
    return ret;
  }
  return T();
}

int main(int argc, char **argv)
{
  bool convert_success;
  uint16_t bla = ConvertString<uint16_t>("15", convert_success);
  if(convert_success)
    std::cout << bla << std::endl;
  else
    std::cerr << "Could not convert" << std::endl;
  return 0;
}