function_extractor.hpp

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/// \file jsonv/serialization/function_extractor.hpp
///
/// Copyright (c) 2015-2026 by Travis Gockel. All rights reserved.
///
/// This program is free software: you can redistribute it and/or modify it under the terms of the Apache License
/// as published by the Apache Software Foundation, either version 2 of the License, or (at your option) any later
/// version.
///
/// \author Travis Gockel (travis@gockelhut.com)
#pragma once
 
#include <jsonv/config.hpp>
#include <jsonv/serialization/extract.hpp>
 
#include "extractor_for.hpp"
 
#include <concepts>
#include <expected>
#include <type_traits>
#include <utility>
 
namespace jsonv
{
 
/// \addtogroup Serialization
/// \{
 
/// An \c extractor which calls a function to perform extraction.
///
/// The function may take any of the shapes \c detail::invoke_extract accepts and may return either a \c T or a
/// \c std::expected<T, ast_node_type>.
template <typename T, typename FExtract>
class function_extractor :
        public extractor_for<T>
{
public:
    // Constrained so that copying from a non-const lvalue reaches the copy constructor, which this would otherwise
    // beat as the better match.
    template <typename FUExtract>
        requires (!std::same_as<std::remove_cvref_t<FUExtract>, function_extractor>)
    explicit function_extractor(FUExtract&& func) :
            _func(std::forward<FUExtract>(func))
    { }
 
protected:
    JSONV_NODISCARD
    virtual std::expected<T, ast_node_type> create(extraction_context& context, reader& from) const override
    {
        return detail::invoke_extract<T>(_func, context, from);
    }
 
private:
    FExtract _func;
};
 
/// Create an \c extractor from \a func, deducing what it extracts from its return type.
template <typename FExtract>
JSONV_NODISCARD
auto make_extractor(FExtract func)
    -> function_extractor<detail::extract_function_result_t<FExtract>, FExtract>
{
    return function_extractor<detail::extract_function_result_t<FExtract>, FExtract>(std::move(func));
}
 
/// \}
 
}