#include <iostream>
#include <stdexcept>
 
#include <boost/format.hpp> // for easy string building
 
int factorial( int n ) {
    if ( n < 0 ) {
        using namespace boost;
        format errmsg( "Can't compute (%d)!" );
        throw std::invalid_argument( (errmsg % n).str() );
    }
 
    int result = 1;
    while ( n != 0 )
        result *= n--;
 
    return result;
}
 
int main() {
    try {
        std::cout << factorial( 3 ) << std::endl;
        std::cout << factorial( 0 ) << std::endl;
        std::cout << factorial( -2 ) << std::endl;
    } catch ( std::invalid_argument &e ) {
        std::cerr << e.what() << std::endl;
    }
}

6
1
Can't compute (-2)!