any_ref.hpp

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// Copyright (C) 2020-2023 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0
 
#ifndef LEXY_DETAIL_ANY_REF_HPP_INCLUDED
#define LEXY_DETAIL_ANY_REF_HPP_INCLUDED
 
#include <lexy/_detail/config.hpp>
 
// Essentially a void*, but we can cast it in a constexpr context.
// The cost is an extra layer of indirection.
 
namespace lexy::_detail
{
template <typename T>
class any_holder;
 
// Store a pointer to this instead of a void*.
class any_base
{
public:
    any_base(const any_base&)            = delete;
    any_base& operator=(const any_base&) = delete;
 
    template <typename T>
    constexpr T& get() noexcept
    {
        return static_cast<any_holder<T>*>(this)->get();
    }
    template <typename T>
    constexpr const T& get() const noexcept
    {
        return static_cast<const any_holder<T>*>(this)->get();
    }
 
private:
    constexpr any_base() = default;
    ~any_base()          = default;
 
    template <typename T>
    friend class any_holder;
};
 
using any_ref  = any_base*;
using any_cref = const any_base*;
 
// Need to store the object in here.
template <typename T>
class any_holder : public any_base
{
public:
    constexpr explicit any_holder(T&& obj) : _obj(LEXY_MOV(obj)) {}
 
    constexpr T& get() noexcept
    {
        return _obj;
    }
    constexpr const T& get() const noexcept
    {
        return _obj;
    }
 
private:
    T _obj;
};
} // namespace lexy::_detail
 
#endif // LEXY_DETAIL_ANY_REF_HPP_INCLUDED