- Single header (https://raw.githubusercontent.com/boost-ext/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/boost-ext/reflect - see examples)
- Verifies itself upon include (can be disabled with
-DNTEST- see FAQ) - Optimized compilation-times (see benchmarks)
- C++20 (clang++13+, g++11+, msvc-19.34+ -Wall)
-Wextra -Werror -pedantic -pedantic-errors | /W4 /WX
// 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(typeid(type_list<int>) == typeid(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)...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/Y3rT58xMq)
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/boost-ext/reflect / https://godbolt.org/z/bPPjY3b3v)
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/crbeMfo3r)
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>})
));
}/**
* 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 run-time info
*
* @code
* info i = meta<conts int>; // run-time
* static_assert(invoke([]<info m> { return std::is_const_v<type_of<m>>; }, i));
* @endcode
*/
constexpr auto invoke(auto fn, 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);Configuration
#define MP 2'0'4 // Current library version (SemVer)
#define MP_SIZE 1024 // Max number of meta types (compilation times optimization)-
What does it mean that
mptests itself upon include?mpruns 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
-DNTESTis defined static_asserts tests won't 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
mpcompares to Reflection for C++26 (https://wg21.link/P2996)?mpmeta-programming model is very simpilar to P2996 and it's based on type erased info object and meta-functions.mpalso supports all C++ standard library and since verion 2.0.0+mptype names have been adopted to closer reflect the reflection proposal.P2996 (C++26*) mp (C++20) ^Tmeta<T>typename [: T :]type_of<T>template for(https://wg21.link/p1306)for_eachsubstituteapplyvalue_of<R>(reflect_invoke(^fn, {substitute(^meta, {reflect_value(m)})}))invoke(fn, m) -
How to integrate with CMake/CPM?
CPMAddPackage( Name mp GITHUB_REPOSITORY boost-ext/mp GIT_TAG v2.0.4 ) add_library(mp INTERFACE) target_include_directories(mp SYSTEM INTERFACE ${mp_SOURCE_DIR}) add_library(mp::mp ALIAS mp)target_link_libraries(${PROJECT_NAME} mp::mp); -
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