Unfinished C++ mini-framework for type-safe JSON construction using CRTP, operator overloading, and generic lambdas.

CRTPJsonDSLBuilder.hpp

// Copyright (c) March 2026 Félix-Olivier Dumas. All rights reserved.
// Licensed under the terms described in the LICENSE file

template<template<typename> class Res, template<typename> class Dis>
struct JsonBuildingPair {
    template<typename T>
    using Resolver = Res<T>;

    template<typename T>
    using Dispatcher = Dis<T>;
};

/********************************************************/

struct sentinel_t {};
inline constexpr sentinel_t sentinel{};

template<typename>
concept requires_derived = true;

struct parentheses;
struct subscript;

/***/

template<typename Operator, auto Behavior, typename CRTP_Context = void>
struct operator_wrapper;

template<auto Behavior>
struct operator_wrapper<parentheses, Behavior> {
    template<typename... Args>
    auto operator()(Args&&... args) {
        return Behavior(std::forward<Args>(args)...);
    }
};

template<auto Behavior, typename CRTP_Context>
struct operator_wrapper<parentheses, Behavior, CRTP_Context> {
    template<typename... Args>
    auto operator()(Args&&... args) {
        return Behavior(
            std::forward<Args>(args)...,
            static_cast<CRTP_Context&>(*this)
        );
    }
};

/**/

template<auto Behavior>
struct operator_wrapper<subscript, Behavior> {
    template<typename S = sentinel_t>
    auto operator[](S&&) { return Behavior(); }

    template<typename... Args>
    auto operator[](Args&&... args) {
        return Behavior(std::forward<Args>(args)...);
    }
};

template<auto Behavior, typename CRTP_Context>
struct operator_wrapper<subscript, Behavior, CRTP_Context> {
    template<typename S = sentinel_t>
    auto operator[](S&&) { return Behavior(static_cast<CRTP_Context&>(*this)); }

    template<typename... Args>
    auto operator[](Args&&... args) {
        return Behavior(
            std::forward<Args>(args)...,
            static_cast<CRTP_Context&>(*this)
        );
    }
};

/*********************************************************************/

struct JsonBuilderResolverTag {};
struct JsonBuilderDispatcherTag {};

/*********************************************************************/

template<typename OperatorTag, auto Behavior, typename CrtpContext>
struct JsonBuilderOperatorBase : operator_wrapper<OperatorTag, Behavior, CrtpContext> {};

/*********************************************************************/

constexpr auto NumberDispatcherBehavior =
    []<typename CrtpContext>(auto val, CrtpContext&& context) {
        return context;
    };

template<typename CrtpContext>
struct JsonBuilderNumberDispatcher : JsonBuilderDispatcherTag,
    public JsonBuilderOperatorBase<parentheses, NumberDispatcherBehavior, CrtpContext>
{
    using Base = JsonBuilderOperatorBase<parentheses, NumberDispatcherBehavior, CrtpContext>;
    auto operator()(auto val) -> decltype(auto) { return Base::operator()(val); }
};

/*********************************************************************/

constexpr auto NumberResolverBehavior =
    []<typename CrtpContext>(CrtpContext&& context) {
        return JsonBuilderNumberDispatcher<CrtpContext>{};
    };

template<typename CrtpContext>
struct JsonBuilderNumberResolver : JsonBuilderResolverTag,
    public JsonBuilderOperatorBase<subscript, NumberResolverBehavior, CrtpContext>
{
    using Base = JsonBuilderOperatorBase<subscript, NumberResolverBehavior, CrtpContext>;
    auto operator[](JSON_TAG_NUMBER) -> decltype(auto) { return Base::operator[](sentinel); }
};

/*********************************************************************/

constexpr auto TableResolverBehavior =
    []<typename CrtpContext>(CrtpContext&& context) {
        std::cout << typeid(CrtpContext).name() << "\n";
        std::cout << context.a << "\n";
        return ChainResolverA{};
    };

template<typename CrtpContext>
struct JsonBuilderTableResolver : JsonBuilderResolverTag,
    public JsonBuilderOperatorBase<subscript, TableResolverBehavior, CrtpContext>
{
    using Base = JsonBuilderOperatorBase<subscript, TableResolverBehavior, CrtpContext>;
    auto operator[](JSON_TAG_TABLE) -> decltype(auto) { return Base::operator[](sentinel); }
};

/*********************************************************************/

template<template<typename> class... Resolvers>
struct JsonBuilderProxy2 : Resolvers<JsonBuilderProxy2<Resolvers...>>... {
    using Resolvers<JsonBuilderProxy2<Resolvers...>>::operator[]...;
};