Re: boost::lexical_cast string to string problem

From: tslettebo1 (terje.s_at_[hidden])
Date: 2002-05-20 21:48:45

--- In Boost-Users_at_y..., "Jason Fischl" <yg-boost-users_at_m...> wrote:
> I'm seeing a problem with lexical_cast. The following code when 
trying to
> convert from string to
> string fails when passed an empty string. The library code that is 
doing
> this lexical_cast does
> not and can not know that the input and output types are the same.
> 
> I believe that this code should not throw.
> 
> #include <iostream>
> #include <boost/lexical_cast.hpp>
> 
> main()
> {
>    std::string input("");
>    try
>    {
>       boost::lexical_cast<std::string>(input);
>    }
>    catch (boost::bad_lexical_cast& e)
>    {
>       std::cout << "Failed" << std::endl;
>    }
> }

Since one can't partially specialise function templates, one way to 
do it could be that the library included something like this:

namespace boost
{
  template<>
  std::string lexical_cast<std::string,std::string>(std::string arg)
  {
  return arg;
  }

  template<>
  std::string lexical_cast<std::string,const char *>(const char *arg)
  {
  return arg;
  }

  template<>
  std::string lexical_cast<std::string,char *>(char *arg)
  {
  return arg;
  }
}

This is quite brittle, though, as it only works for the types given. 
It does work for your example, for Intel C++ and VC++, though (the 
compilers I've tested it with).

If partial specialisation was possible, you could do:

namespace boost
{
  template<class T>
  T lexical_cast<T,T>(T arg)
  {
  return arg;
  }
}

Interestingly, the person behind lexical_cast, Kevlin Henney, is on 
the standards committee, and in this case, a language change could 
help his library. :)

Regards,

Terje