evaluateIfPresent.hh

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
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/*   This file is part of the C++-library RFFGen.                              */
/*   Copyright 2015 Lars Lubkoll                                               */
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/*   RFFGen is free software: you can redistribute it and/or modify            */
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#ifndef RFFGEN_UTIL_EVALUATE_IF_PRESENT_HH
#define RFFGEN_UTIL_EVALUATE_IF_PRESENT_HH

#include <type_traits>

namespace RFFGen
{
  namespace Detail
  {
    template <class X, bool Xpresent>
    struct EvaluateIfPresentImpl
    {
      static constexpr bool present = false;
      EvaluateIfPresentImpl(X const&){}
    };

    template <class X>
    struct EvaluateIfPresentImpl<X,true>
    {
      using ReturnType = std::add_const_t<decltype(std::declval<X>()())>;
      static constexpr bool present = true;

      EvaluateIfPresentImpl(X const& x) : value(x())
      {}

      ReturnType operator()() const
      {
        return value;
      }

      ReturnType value;
    };
  }


  template <class X>
  struct EvaluateIfPresent : public Detail::EvaluateIfPresentImpl<X,X::present>
  {
    EvaluateIfPresent(X const& x) : Detail::EvaluateIfPresentImpl<X,X::present> (x)
    {}

    template <class... Args>
    EvaluateIfPresent(const Args&... args) : EvaluateIfPresent( X(args...) )
    {}
  };
}

#endif // RFFGEN_UTIL_EVALUATE_IF_PRESENT_HH