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C++ single header/single module C++20 Meta-Programming Library
Motivation
Make
TemplateMeta-Programming easier by leveraging run-time approach at compile-time. If one knows how to use stl.algorithms/ranges one can consider themself a TMP expert now as well!
#include <ranges>
auto hello_world = [](auto list, auto add_const, auto has_value){
return list // int, foo, val, bar
| std::views::drop(1_c) // foo, val, bar
| std::views::reverse // bar, val, foo
| std::views::take(2_c) // bar, val
| std::views::transform(add_const) // bar const, val const
| std::views::filter(has_value) // val const
;
};auto add_const = []<class T> -> T const {};
auto has_value = []<class T> { return requires(T t) { t.value; }; };struct bar {};
struct foo { int value; };
struct val { int value; };static_assert(mp::list<val const> ==
hello_world(mp::list<int, foo, val, bar>, add_const, has_value)
);int main () {
struct stub_type{
std::size_t type{};
bool has_value{};
bool add_const{};
constexpr auto operator<=>(const stub_type&) const = default;
};
"hello world"_test = [] {
// given
std::vector list{stub_type{.type = 0},
stub_type{.type = 1, .has_value = true}};
auto add_const = [](auto& t) { t.add_const = true; return t; };
auto has_value = [](auto t) { return t.has_value; };
// when
auto out = hello_world(list, add_const, has_value);
const std::vector<stub_type> result{std::begin(out), std::end(out)};
// then
expect(1_u == std::size(result));
expect(result[0] == stub_type{.type = 1,
.has_value = true,
.add_const = true});
};
}// write once, use multiple times
auto slice = [](auto list, auto begin, auto end) {
return list
| std::views::drop(begin)
| std::views::take(end - 1_c)
;
};// type_list
static_assert(slice(mp::list<int, double, float, short>, 1_c, 3_c) ==
mp::list<double, float>);// variant.type
static_assert(std::is_same_v<
mp::typeof<slice, std::variant<int, double, float, short>, 1_c, 3_c>,
std::variant<double, float>>
);// value_list
static_assert(slice(mp::list<1, 2, 3, 4>, 1_c, 3_c) ==
mp::list<2, 3>);// fixed_string
static_assert(slice(mp::list<"foobar">, 1_c, 3_c) ==
mp::list<"oo">);// tuple of values
static_assert(slice(std::tuple{1, 2, 3, 4}, 1_c, 3_c) ==
std::tuple{2, 3});#include <cassert>
int main(int argc, const char**) {
// run-time tuple of values
assert((slice(std::tuple{1, argc, 3, 4}, 1_c, 3_c) ==
std::tuple{argc, 3}));
}auto fun_with_tuple = [](auto tuple) {
return tuple
| std::views::filter([](auto i) -> bool { return i % 2; })
| std::views::reverse
| std::views::drop(1_c)
;
};
static_assert(std::tuple{5, 3, 1} == fn([] { return std::tuple{1, 2, 3, 4, 5, 6, 7}; }));#include <algorithm>
auto sort_by_size = [](std::ranges::range auto types) {
std::ranges::sort(types, [](auto lhs, auto rhs) { return lhs.size < rhs.size; });
return types;
};
/**
* Verify/debug at run-time
*/
int main () {
"sort by size"_test = [] {
// given
const auto m1 = meta{.index = 0, .size = 2};
const auto m2 = meta{.index = 1, .size = 1};
const auto m3 = meta{.index = 2, .size = 3};
// when
const auto sorted = sort_by_size({m1, m2, m3});
// then
expect({m2, m1, m3} == sorted);
};
}struct not_packed {
char c{};
int i{};
std::byte b{};
};
/**
* Check at compile-time
*/
static_assert(sizeof(not_packed) == 12u);
static_assert(sizeof(to_tuple(not_packed{}) | sort_by_size) == 8u);#include <ranges>
#include <algorithm>
auto rotate = [](std::ranges::range auto types) {
std::ranges::rotate(types, std::begin(types) + 1);
return types;
};
static_assert((boost::mp::list<int, double, float>() | rotate) ==
boost::mp::list<double, float, int>());Quick Start
Try it out - https://godbolt.org/z/ePE9aqYTe
Locally
docker build . -t dev # or docker pull krisjusiak/dev
docker run -it -v "$(pwd)":/mp --privileged dev:latestmkdir build && cd build
CXX={clan}g++ cmake .. -DBOOST_MP_BUILD_TESTS=ON -DBOOST_MP_BUILD_EXAMPLES=ONcmake --build . -jctest --output-on-failureOverview
- Single C++20 header/module
- Minimal learning curve (reuses STL, ranges or any third-party algorithms for stl.container)
- Easy debugging (meta-functions can be simply run at run-time!)
- Same interface for
types/values/tuples - Declarative by design (composable using pipe operator, support for ranges)
- Fast compilation times (see benchmarks)
Requirements (Dockerfile)
- C++20 compliant compiler (STL with support for
constexpr std::vector)- clang++15+ [libc++-15+] (✔️)
- g++12+ [libstdc++-12+] (✔️)
Tutorial
Firstly include or import
boost.mp
#include <boost/mp.hpp>or
import boost.mp;Okay, let's write a hello world, shall we?
First step is to add our new meta-function.
auto identity = [](std::ranges::range types) {
return types;
};meta is a meta objects range (like vector<meta>) which we can do operations on.
For example, sorting, changing the size, removing elements, etc...
Let's apply our first meta-function.
auto magic = mp::list<int, double, float>() | identity;static_assert(magic == mp::list<int, double, float>());Yay, we have the first meta-function done. Notice the pipe (|) operator. By using it multiple meta-functions can be combined together.
For the next example including/importing ranges will be required
#include <ranges>Let's implement simple slice for types as an example
template<auto list, auto Start, auto End>
auto slice = list
| std::views::drop(Start)
| std::views::take(End);Notice that we've just used std::ranges at compile-time to manipulate a type-list!
using mp::operator""_c;
static_assert(slice<mp::list<int, double, float>(), 1_c, 2_c>
== mp::list<double, float>());""_c is an User Defined Literal which represents constant integral value which
is required for simulating passing constexpr parameters which aren't supported
in C++.
Let's add STL too, why not
#include <tuple>
#include <algorithm>
This time we will sort and reverse a tuple
Note: All operations are supported for the following entities
mp::type_listmp::value_listmp::fixed_stringstd::tuple
Additionally type_list/value_list/fixed_string will be deduced automatically
based on parameters when mp::list<...>() is used.
Okay, coming back to our sort...
template <auto Fn>
auto sort = [](std::ranges::range auto types) {
std::sort(std::begin(types), std::end(types), Fn);
return types;
};
> Note With ranges that could be `actions::sort(types, Fn)`
auto by_size = [](auto lhs, auto rhs) { return lhs.size < rhs.size; };So far, nothing magical, same code as in run-time!
Let's apply it then
using mp::operator|;
auto pack = [](auto t) {
return mp::to_tuple(t) | sort<by_size>;
}Note: We used
to_tuplewhich converts a struct into a tuple using reflection. There is alsoto_listavailable which producestype_list.
As usual, we use pipe (|) to compose functionality.
struct not_packed {
char c{}; // 1b
int i{}; // 4b
std::byte b{}; // 1b
};static_assert(sizeof(not_packed) == 12u);
static_assert(sizeof(pack(not_packed{})) == 8u);Okay, so far so good, but what about adding or removing from type_list?
Removing is simple as we can just erase elements from the meta types as before.
Dealing with new types it's a bit different but not difficult either.
template <auto List, class... Ts>
auto add = List | mp::list<Ts...>();static_assert(add<mp::list<int, double>(), void> ==
mp::list<int, double, void>());And what about transform? Let's add pointers to all our type in the list.
auto transform = [](auto list){
return list | std::views::transform([]<class T> -> Ts* const {})
};It's that easy, we just apply addition of const pointer to all Ts....
static_assert(transform(mp::list<int, double>) ==
mp::list<int* const, double* const>);Okay, so what about the case when we need meta-types and Ts...?
auto filter = std::views::filterclass T> { return requires(T t) { t.value; }; });Notice handy
requires with lambdapattern to verify ad-hoc concepts.
struct bar {};
struct foo {
int value;
};static_assert(mp::list<foo>() ==
(mp::list<foo, bar>() | filter));That's it for now, for more let's take a look at more Examples in the following section and the User-Guide.
Examples
User-Guide
/**
* Library version for example 1'0'0
*/
#define BOOST_MP_VERSION/**
* Forces using includes even if modules are supported
*/
#define BOOST_MP_DISABLE_MODULE/**
* A meta type representation to be manipulated
* vector<meta> is passed to pipe lambdas
*/
struct meta {
std::size_t pos{};
std::size_t size{};
//...
}/**
* Returns unique integral (std::size_t) representation of type
* Should only be used for comparison
* static_assert(type_id<void> ! = type_id<int>);
*/
template <class T> constexpr auto type_id;/**
* Returns type/value name as string_view
* static_assert(type_name<void>() == "void");
* static_assert(type_name<42>() == "42");
*/
template <template auto T> [[nodiscard]] constexpr auto type_name()/**
* A meta concept which verifies meta list
* static_assert(concepts::meta<list<>>>);
* static_assert(concepts::meta<list<int, double>>>);
* static_assert(concepts::meta<std::tuple<int, double>>>);
*/
concept concepts::meta;/**
* List of types
*/
template<class... Ts> struct type_list {
constexpr auto size = sizeof...(Ts);
constexpr operator==(type_list) = default;
constexpr operator[](auto N); // returns N-th type
};/**
* List of values
*/
template<class... Ts> struct value_list;
constexpr auto size = sizeof...(Vs);
constexpr operator==(value_list) = default;
constexpr operator[](auto N); // returns N-th value
};/**
* Compile-time string representation to be used
* as <"Hello World">
*/
template<std::size_t N> struct fixed_string {
static constexpr auto size = N;
[[nodiscard]] constexpr auto operator<=>(const fixed_string&) const = default;
[[nodiscard]] constexpr explicit(false) operator std::string_view() const;
std::array<char, N + 1> data{};
};/**
* Deduces correct list based on types
* type_list for Ts...
* value_list for auto...
* fixed_string for if { t.data; t.size; }
*/
template<template auto... Vs> [[nodiscard]] constexpr auto list();/**
* Converts type into a type_list by reflecting fields
* 0-10 number of reflected fields is supported
* @tparam T type to be reflected
*/
template<class T> [[nodiscard]] constexpr auto to_list;/**
* Converts type into a std::tuple by reflecting fields
* 0-10 number of reflected fields is supported
* @param obj object to be reflected
*/
constexpr auto to_tuple = []<class T>(T&& obj);/**
* Composability pipe operator for types
* @param fn functor to be applied
* - [](concepts::meta auto types)
* - []<class... Ts>
* - []<class... Ts>(concepts::meta auto types)
*/
template <template <class...> class T, class... Ts>
[[nodiscard]] constexpr auto operator|(T<Ts...>, auto fn);/**
* Composability pipe operator for values
* @param fn functor to be applied
* - [](concepts::meta auto types)
* - []<auto... Ts>
* - []<auto... Ts>(concepts::meta auto types)
*/
template <template <auto...> class T, auto... Vs>
[[nodiscard]] constexpr auto operator|(T<Vs...>, auto fn);/**
* Composability pipe operator for std::tuple
* @param fn functor to be applied
* - [](concepts::meta auto types)
* - [](auto&&... args)
* - [](concepts::meta auto types, auto&&... args)
*/
template <template <class...> class T, class... Ts>
[[nodiscard]] constexpr auto operator|(std::tuple<Ts...>, auto fn);/**
* Compile time integral value representation (required to mimic constexpr parameters)
* static_assert(42 == _c(1+41));
* static_assert(42 == 42_c);
*/
template <auto N> constexpr auto _c;
template <char... Cs> [[nodiscard]] consteval auto operator""_c();Benchmarks
To build/run benchmarks
cd benchmark
mkdir build && cd build
CXX={clang}g++ cmake ..
make <<benchmark>>Disclaimer MP is not an official Boost library.
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