c++ - Partial template specialization based on "signed-ness" of integer type?

Question

Given:

template<typename T>
inline bool f( T n ) {
  return n >= 0 && n <= 100;
}   

When used with an unsigned type generates a warning:

unsigned n;
f( n ); // warning: comparison n >= 0 is always true

Is there any clever way not to do the comparison n >= 0 when T is an unsigned type? I tried adding a partial template specialization:

template<typename T>
inline bool f( unsigned T n ) {
  return n <= 100;
}   

but gcc 4.2.1 doesn't like that. (I didn't think that kind of partial template specialization would be legal anyway.)

Answer 1

You can use enable_if with the is_unsigned type trait:

template <typename T>
typename std::enable_if<std::is_unsigned<T>::value, bool>::type f(T n)
{
    return n <= 100;  
}

template <typename T>
typename std::enable_if<!std::is_unsigned<T>::value, bool>::type f(T n)
{
    return n >= 0 && n <= 100;  
}

You can find enable_if and is_unsigned in the std or std::tr1 namespaces if your compiler supports C++0x or TR1, respectively. Otherwise, Boost has an implementation of the type traits library, Boost.TypeTraits. The boost implementation of enable_if is a little different; boost::enable_if_c is similar to the TR1 and C++0x enable_if.