invoke.h

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#include <functional>
#include <type_traits>
#include <utility>

namespace Gaudi
{
  namespace detail2
  {

    // An implemention of std::invoke based on
    // http://en.cppreference.com/w/cpp/utility/functional/invoke
    template <class T>
    struct is_reference_wrapper : std::false_type {
    };
    template <class U>
    struct is_reference_wrapper<std::reference_wrapper<U>> : std::true_type {
    };

    template <class Base, class T, class Derived, class... Args>
    auto INVOKE( T Base::*pmf, Derived&& ref, Args&&... args ) noexcept(
        noexcept( ( std::forward<Derived>( ref ).*pmf )( std::forward<Args>( args )... ) ) )
        -> std::enable_if_t<std::is_function<T>::value && std::is_base_of<Base, std::decay_t<Derived>>::value,
                            decltype( ( std::forward<Derived>( ref ).*pmf )( std::forward<Args>( args )... ) )>
    {
      return ( std::forward<Derived>( ref ).*pmf )( std::forward<Args>( args )... );
    }

    template <class Base, class T, class RefWrap, class... Args>
    auto INVOKE( T Base::*pmf, RefWrap&& ref,
                 Args&&... args ) noexcept( noexcept( ( ref.get().*pmf )( std::forward<Args>( args )... ) ) )
        -> std::enable_if_t<std::is_function<T>::value && is_reference_wrapper<std::decay_t<RefWrap>>::value,
                            decltype( ( ref.get().*pmf )( std::forward<Args>( args )... ) )>

    {
      return ( ref.get().*pmf )( std::forward<Args>( args )... );
    }

    template <class Base, class T, class Pointer, class... Args>
    auto INVOKE( T Base::*pmf, Pointer&& ptr, Args&&... args ) noexcept(
        noexcept( ( ( *std::forward<Pointer>( ptr ) ).*pmf )( std::forward<Args>( args )... ) ) )
        -> std::enable_if_t<std::is_function<T>::value && !is_reference_wrapper<std::decay_t<Pointer>>::value &&
                                !std::is_base_of<Base, std::decay_t<Pointer>>::value,
                            decltype( ( ( *std::forward<Pointer>( ptr ) ).*pmf )( std::forward<Args>( args )... ) )>
    {
      return ( ( *std::forward<Pointer>( ptr ) ).*pmf )( std::forward<Args>( args )... );
    }

    template <class Base, class T, class Derived>
    auto INVOKE( T Base::*pmd, Derived&& ref ) noexcept( noexcept( std::forward<Derived>( ref ).*pmd ) )
        -> std::enable_if_t<!std::is_function<T>::value && std::is_base_of<Base, std::decay_t<Derived>>::value,
                            decltype( std::forward<Derived>( ref ).*pmd )>
    {
      return std::forward<Derived>( ref ).*pmd;
    }

    template <class Base, class T, class RefWrap>
    auto INVOKE( T Base::*pmd, RefWrap&& ref ) noexcept( noexcept( ref.get().*pmd ) )
        -> std::enable_if_t<!std::is_function<T>::value && is_reference_wrapper<std::decay_t<RefWrap>>::value,
                            decltype( ref.get().*pmd )>
    {
      return ref.get().*pmd;
    }

    template <class Base, class T, class Pointer>
    auto INVOKE( T Base::*pmd, Pointer&& ptr ) noexcept( noexcept( ( *std::forward<Pointer>( ptr ) ).*pmd ) )
        -> std::enable_if_t<!std::is_function<T>::value && !is_reference_wrapper<std::decay_t<Pointer>>::value &&
                                !std::is_base_of<Base, std::decay_t<Pointer>>::value,
                            decltype( ( *std::forward<Pointer>( ptr ) ).*pmd )>
    {
      return ( *std::forward<Pointer>( ptr ) ).*pmd;
    }

    template <class F, class... Args>
    auto INVOKE( F&& f, Args&&... args ) noexcept( noexcept( std::forward<F>( f )( std::forward<Args>( args )... ) ) )
        -> std::enable_if_t<!std::is_member_pointer<std::decay_t<F>>::value,
                            decltype( std::forward<F>( f )( std::forward<Args>( args )... ) )>
    {
      return std::forward<F>( f )( std::forward<Args>( args )... );
    }
  } // namespace detail2

  template <class F, class... ArgTypes>
  auto invoke( F&& f, ArgTypes&&... args )
      // exception specification for QoI
      noexcept( noexcept( detail2::INVOKE( std::forward<F>( f ), std::forward<ArgTypes>( args )... ) ) )
          -> decltype( detail2::INVOKE( std::forward<F>( f ), std::forward<ArgTypes>( args )... ) )
  {
    return detail2::INVOKE( std::forward<F>( f ), std::forward<ArgTypes>( args )... );
  }
}