pair.hpp

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#pragma once
 
namespace flecs {
 
namespace _ {
    struct pair_base { };   
} // _
 
 
template <typename First, typename Second>
struct pair : _::pair_base { 
    using type = conditional_t<!is_empty<First>::value || is_empty<Second>::value, First, Second>;
    using first = First;
    using second = Second;
 
    pair(type& v) : ref_(v) { }
 
    // This allows the class to be used as a temporary object
    pair(const type& v) : ref_(const_cast<type&>(v)) { }
 
    operator type&() { 
        return ref_;
    }
 
    operator const type&() const { 
        return ref_;
    }    
 
    type* operator->() {
        return &ref_;
    }
 
    const type* operator->() const {
        return &ref_;
    }
 
    type& operator*() {
        return ref_;
    }
 
    const type& operator*() const {
        return ref_;
    }
    
private:
    type& ref_;
};
 
template <typename First, typename Second, if_t<is_empty<First>::value> = 0>
using pair_object = pair<First, Second>;
 
 
template <typename T>
struct is_pair {
    static constexpr bool value = is_base_of<_::pair_base, remove_reference_t<T> >::value;
};
 
 
template <typename P>
using pair_first_t = transcribe_cv_t<remove_reference_t<P>, typename remove_reference_t<P>::first>;
 
template <typename P>
using pair_second_t = transcribe_cv_t<remove_reference_t<P>, typename remove_reference_t<P>::second>;
 
template <typename P>
using pair_type_t = transcribe_cv_t<remove_reference_t<P>, typename remove_reference_t<P>::type>;
 
template <typename T, typename U = int>
struct actual_type;
 
template <typename T>
struct actual_type<T, if_not_t< is_pair<T>::value >> {
    using type = T;
};
 
template <typename T>
struct actual_type<T, if_t< is_pair<T>::value >> {
    using type = pair_type_t<T>;
};
 
template <typename T>
using actual_type_t = typename actual_type<T>::type;
 
 
// Get type without const, *, &
template<typename T>
struct base_type {
    using type = decay_t< remove_pointer_t< actual_type_t<T> > >;
};
 
template <typename T>
using base_type_t = typename base_type<T>::type;
 
 
// Get type without *, & (retains const which is useful for function args)
template<typename T>
struct base_arg_type {
    using type = remove_pointer_t< remove_reference_t< actual_type_t<T> > >;
};
 
template <typename T>
using base_arg_type_t = typename base_arg_type<T>::type;
 
 
// Test if type is the same as its actual type
template <typename T>
struct is_actual {
    static constexpr bool value = 
        std::is_same<T, actual_type_t<T> >::value && !is_enum<T>::value;
};
 
} // flecs