[Boost-users] [lexical_cast] Input/ouput to stringstream ok, but not with lexical_cast

From: François Duranleau (duranlef_at_[hidden])
Date: 2006-03-15 18:03:50

Hi!

If I read the documentation correctly about boost::lexical_cast, for a 
convertion between two types to work, the source type must be 
OutputStreamable and the target type must be InputStreamable (plus some 
other requirements about copying and default construction).

Then, any idea why the following code does not work?

/////////////////////////////////////////////////////////////////////////
#include <boost/lexical_cast.hpp>
#include <iostream>

using namespace std ;

struct vec
{
     enum static_size_type { size = 2 } ;
     double data[ size ] ;

     vec()
     {
         for ( int i = 0 ; i < size ; ++ i ) data[ i ] = 0.0 ;
     }
} ;

istream& operator >> ( istream& in , vec& v )
{
     for ( int i = 0 ; i < vec::size ; ++ i ) in >> v.data[ i ] ;
     return in ;
}

ostream& operator << ( ostream& out , const vec& v )
{
     out << v.data[ 0 ] ;
     for ( int i = 1 ; i < vec::size ; ++ i ) out << ' ' << v.data[ i ] ;
     return out ;
}

int main()
{
     string arg = "1 2" ;
     try
     {
         vec v2 = boost::lexical_cast< vec >( arg ) ;
         cerr << "lexical_cast: " << v2 << endl ;
     }
     catch ( const boost::bad_lexical_cast& e )
     {
         cerr << e.what() << endl ;
     }
     return 0 ;
}
/////////////////////////////////////////////////////////////////////////

I use g++ 4.0.2 and Boost 1.33.1. The program compiles, but when executed, 
I get a boost::bad_lexical_cast exception. However, if I change vec::size 
to 1 and the variable arg in the main function to "1", then everything 
runs fine. Any other values greater than 1 won't work (with an appropriate 
source string, of course).

There is an example a little more detailed here:
http://www-etud.iro.umontreal.ca/~duranlef/code/lexical_cast_problem.cpp

In that example, I manually output the string to a stringstream and then 
input a vec from the stream, check that I at the end of the stream, and 
everything runs ok, but not with the lexical cast. I guess I must be doing 
something stupid somewhere, but I don't see it.

Thanks for any help,

-- 
François Duranleau
LIGUM, Université de Montréal
"Conflict may erupt at any time and any place. War is the destiny of which
  humanity can never wash its hands."
                          - Emperor Dornkirk, in _The Vision of Escaflowne_