ifthenelse.hpp

The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017


#ifndef IFTHENELSE_HPP
#define IFTHENELSE_HPP

// primary template: yield the second argument by default and rely on
//      a partial specialization to yield the third argument
//      if COND is false
template<bool COND, typename TrueType, typename FalseType>
struct IfThenElseT {
    using Type = TrueType;
};

// partial specialization: false yields third argument
template<typename TrueType, typename FalseType>
struct IfThenElseT<false, TrueType, FalseType> {
    using Type = FalseType;
};

template<bool COND, typename TrueType, typename FalseType>
using IfThenElse = typename IfThenElseT<COND, TrueType, FalseType>::Type;
#endif // IFTHENELSE_HPP