// Overview / Examples / API / FAQ

MP: Template Meta-Programming library

https://en.wikipedia.org/wiki/Template_metaprogramming

Features

• Single header (https://raw.githubusercontent.com/qlibs/mp/main/mp - for integration see FAQ)
• Minimal API and learning curve (supports STL, ranges, ...)
• Supports debugging (meta-functions can be executed and debugged at run-time - see examples)
• Supports reflection (requires https://github.com/qlibs/reflect - see examples)
• Verifies itself upon include (can be disabled with -DNTEST - see FAQ)
• Optimized compilation-times (see benchmarks)

Requirements

• C++20 (clang++13+, g++11+, msvc-19.34+)

---

Overview

API (https://godbolt.org/z/zTdYGvKKW)

// mp::meta
static_assert(mp::meta<int> == mp::meta<int>);
static_assert(mp::meta<int> != mp::meta<void>);
static_assert(typeid(mp::meta<int>) == typeid(mp::meta<void>));

// mp::type_of
constexpr mp::info meta = mp::meta<int>;
mp::type_of<meta> i{}; // same as int i{};
mp::type_of<mp::meta<bool>> b = true; // same as bool b = true;

// mp::apply
template<class...> struct type_list{ };
static_assert(std::is_same_v<type_list<int>, mp::apply_t<type_list, std::array{meta}>>);

// mp::invoke
static_assert(not mp::invoke<std::is_const>(meta));
static_assert(std::is_same_v<const int, mp::type_of<mp::invoke<std::add_const>(meta)>>);

int main() {
  // mp::for_each
  constexpr auto v = mp::vector{meta};
  mp::for_each<v>([&]<mp::info meta>{ /* ... */ });
}

// and more (see API)...

---

Examples

Hello World (https://godbolt.org/z/69jGzqPs1)

template<size_t N, class... Ts>
using at_c = mp::type_of<std::array{mp::meta<Ts>...}[N]>;

static_assert(std::is_same_v<int, at_c<0, int, bool, float>>);
static_assert(std::is_same_v<bool, at_c<1, int, bool, float>>);
static_assert(std::is_same_v<float, at_c<2, int, bool, float>>);

---

Algorithms (https://godbolt.org/z/GvzjvdPq8)

template<class... Ts>
struct example {
  mp::apply_t<std::variant,
      std::array{mp::meta<Ts>...}
    | std::views::drop(1)
    | std::views::reverse
    | std::views::filter([](auto m) { return mp::invoke<std::is_integral>(m); })
    | std::views::transform([](auto m) { return mp::invoke<std::add_const>(m); })
    | std::views::take(2)
    | std::ranges::to<mp::vector<mp::info>>()
  > v;
};

static_assert(
  typeid(std::variant<const int, const short>)
  ==
  typeid(example<double, void, const short, int>::v)
);

---

Reflection - https://github.com/qlibs/reflect (https://godbolt.org/z/qb37G79Ya)

struct foo {
  int a;
  bool b;
  float c;
};

constexpr foo f{.a = 42, .b = true, .c = 3.2f};

constexpr mp::vector<mp::info> v =
    members(f)
  | std::views::filter([&](auto meta) { return member_name(meta, f) != "b"; })
  ;

static_assert(std::tuple{42, 3.2f} == to<std::tuple, v>(f));

---

Run-time testing/debugging (https://godbolt.org/z/vTfGGToa4)

constexpr auto reverse(std::ranges::range auto v) {
  std::reverse(v.begin(), v.end());
  return v;
}

int main() {
  static_assert(
    std::array{mp::meta<float>, mp::meta<double>, mp::meta<int>}
    ==
    reverse(std::array{mp::meta<int>, mp::meta<double>, mp::meta<float>})
  );

  assert((
    std::array{mp::meta<float>, mp::meta<double>, mp::meta<int>}
    ==
    reverse(std::array{mp::meta<int>, mp::meta<double>, mp::meta<float>})
  ));
}

---

API

/**
 * Meta info type
 */
enum class info : size_t { };

/**
 * Creates meta type
 *
 * @code
 * static_assert(meta<void> == meta<void>);
 * static_assert(meta<void> != meta<int>);
 * @endcode
 */
template<class T> inline constexpr info meta = /* unspecified */;

/**
 * Returns underlying type from meta type
 *
 * @code
 * static_assert(typeid(type_of<meta<void>>) == typeid(void));
 * @endcode
 */
template<info meta> using type_of = /* unspecified */;

/**
 * Applies invocable `[] { return vector<info>{...}; }` to
 *                   `T<type_of<info>...>`
 *
 * @code
 * static_assert(typeid(variant<int>) ==
 *               typeid(apply<variant>([] { return vector{meta<int>}; })));
 * @endcode
 */
template<template<class...> class T>
[[nodiscard]] constexpr auto apply(concepts::invocable auto expr);

/**
 * Applies range to `T<type_of<info>...>`
 *
 * @code
 * static_assert(typeid(variant<int>) ==
 *               typeid(apply<variant, vector{meta<int>}>));
 * @endcode
 */
template<template<class...> class T, concepts::range auto range>
inline constexpr auto apply_v = decltype(apply<T, [] { return range; }>);

/**
 * Applies range to `T<type_of<info>...>`
 *
 * @code
 * static_assert(typeid(variant<int>) ==
 *               typeid(apply_t<variant, [] { return vector{meta<int>}; }>));
 * @endcode
 */
template<template<class...> class T, concepts::range auto range>
using apply_t = decltype(apply_v<T, range>);

/**
 * Invokes function with compile-time info based on meta-info
 *
 * @code
 * static_assert(invoke(
 *   []<info m> { return std::is_const_v<type_of<m>>;
 * }, meta<conts int>));
 * @endcode
 */
[[nodiscard]] constexpr auto invoke(auto&& fn, info meta);

/**
 * Invokes type_trait with meta-info
 *
 * @code
 * static_assert(not invoke<std::is_const>(meta<int>));
 * static_assert(invoke<std::is_const>(meta<const int>));
 * @endcode
 */
template<template<class...> class T, class... Ts>
[[nodiscard]] constexpr auto invoke(info meta) {

/**
 * Iterates over all elements of a range
 *
 * @code
 * constexpr vector v{meta<int>};
 * for_each<v>([]<info m> {
 *   static_assert(typeid(int) == typeid(type_of<m>));
 * });
 * @endcode
 */
template<concepts::range auto range>
constexpr auto for_each(auto&& fn);

/**
 * Asserts that condition is satisfied
 *  - constant evaluation: calls undefined function `failed()`
 *  - run-time evaluation: #ifndef NDEBUG calls `breakpoint()` otherwise empty call
 */
constexpr void expect(auto cond) requires requires { bool(cond); };

---

FAQ

• What does it mean that mp tests itself upon include?

  mp runs all tests (via static_asserts) upon include. If the include compiled it means all tests are passing and the library works correctly on given compiler, enviornment.

• How to disable running tests at compile-time?

  When -DNTEST is defined static_asserts tests wont be executed upon include. Note: Use with caution as disabling tests means that there are no gurantees upon include that given compiler/env combination works as expected.

• How does it work?

  Slides - Template-less Metaprogramming

  Source code - https://godbolt.org/z/Kf9rovaqE

  • ~100 LOC, C++17, gcc, clang, msvc, no dependencies

• How mp compares to Reflection for C++26 (https://wg21.link/P2996)?

  mp meta-programming model is very simpilar to P2996 and its based on type erased info object and meta-functions. mp also supports all C++ standard library and since verion 2.0.0+ mp type names have been adopted to closer reflect the reflection proposal.

  mp (C++20)	P2996 (C++26*)
  meta<T>	^T
  using info = decltype(meta<void>)	using info = decltype(^::)
  type_of<T>	typename [: T :]
  for_each	template for (https://wg21.link/p1306)
  apply_t	substitute
  invoke<Fn>	reflect_invoke
  invoke<Trait>	test_trait
  invoke<T>	extract

• How to integrate with CMake.FetchContent?

  include(FetchContent)
  
  FetchContent_Declare(
    qlibs.mp
    GIT_REPOSITORY https://github.com/qlibs/mp
    GIT_TAG v2.0.10
  )
  
  FetchContent_MakeAvailable(qlibs.mp)
  

  target_link_libraries(${PROJECT_NAME} PUBLIC qlibs.mp);
  

• Similar projects?

  boost.mp11, boost.hana, boost.fusion, boost.mpl