Jln.mp is a single header C++17 metaprogramming library designed for fast compilation speed (strongly inspired by Kvasir::mpl).

Licence: MIT

Online documentation: https://jonathanpoelen.github.io/jln.mp/

Online documentation for v1: https://jonathanpoelen.github.io/jln.mp/v1/

Single file version in standalone branch.

Concepts

Functions of jln.mp are used in 2 stages:

• Instantiation, by indicating parameters which describe how it must work.
• Call, by applying sequence to an instantiation.

For example, suppose we want to remove void from a sequence. The function to use is jln::mp::remove:

using remove_void = jln::mp::remove<void>;

We can then apply it to our data:

using result = jln::mp::call<remove_void, int, void, double, char>;
// result == jln::mp::list<int, double, char>

Now suppose that result must be a std::tuple. Rather than linking with another function, it is possible to combine them in remove_void via a continuation (C parameter).

using remove_void = jln:mp::remove<void, /*C=*/jln::mp::lift<std::tuple>>;

using result = jln::mp::call<remove_void, int, void, double, char>;
// result == std::tuple<int, double, char>

The default continuations are jln::mp::listify which transforms a sequence into a jln::mp::list and jln::mp::identity which returns the input value.

Jln.mp also has 2 additional namespaces:

• smp a SFINAE compatible version of mp.
• emp which directly returns a result without going through jln::mp::call. According to the function, type sequences are replaced by lists and continuations are not always available.

Create a function

A function is a type with a f template member.

struct to_tuple
{
  template<class... xs>
  using f = std::tuple<xs...>;
};

jln::mp::call<to_tuple, int, double> == std::tuple<int, double>

In the mind of the library, functions should at least take a continuation.

// equivalent to jln::mp::lift<std::tuple, C>
template<class C = jln::mp::identity>
struct to_tuple
{
  template<class... xs>
  using f = jln::mp::call<C, std::tuple<xs...>>;
};

jln::mp::call<to_tuple<>, int, double> == std::tuple<int, double>

Glossary

• Sequence: a value sequence or a type sequence.

• Set: a sequence of unique elements.

• Map: a sequence of lists having at least one element (the key). The keys of the map must be unique.

• Value: a type with a value member.

• Typelist: an instance compatible with template<class...> class T, such as list<>.

• Function: a type with a f template member. The number and the nature of the parameters depend on the context of use.

• Predicate: a function which takes n argument (usually 1) and returns a boolean.

• Meta-function: a template class template<class...> class M.

• Lazy meta-function: a meta-function with a type member.

• C: Continuation function. Represents the function used to chain calls, typically listify or identity.

• TC: True Continuation function. Represents a continuation used when something is found.

• FC: False Continuation function. Represents a continuation used when something is not found.

• _v suffix: C::f takes values. Usually C::f<jln::mp::int_...> (C++17) or C::f<auto...> (C++20). In the emp namespace, with a few exceptions, this corresponds to a variable template (as for the stl).

• _c suffix: number type parameters are jln::mp::int_. Usually foo_c<int_ i> = foo<number<i>>.

Example of real life

Implementation of std::tuple_cat that works with tuple like.

#include "jln/mp/algorithm/make_int_sequence.hpp"
#include "jln/mp/algorithm/transform.hpp"
#include "jln/mp/algorithm/repeat.hpp"
#include "jln/mp/functional/each.hpp"
#include "jln/mp/functional/lift.hpp"
#include "jln/mp/list/join.hpp"

#include <array>
#include <tuple>

namespace mp = jln::mp;
namespace emp = jln::mp::emp;

template<class Tuple>
struct my_tuple_element
{
    template<class I>
    using f = std::tuple_element_t<I::value, Tuple>;
};

template<class... Tuples>
using my_tuple_cat_result_type = mp::call<
    // Convert a sequence of mp::list to std::tuple
    mp::join<mp::lift<std::tuple>>,
    // Convert a tuple like to mp::list of tuple element.
    // To support tuple-likes, it is necessary to use std::tuple_size and std::tuple_element.
    // Otherwise, emp::unpack<Tuples> is sufficient.
    emp::make_int_sequence<
        std::tuple_size<std::decay_t<Tuples>>,
        // Convert a sequence of tuple index to a mp::list of tuple element.
        mp::transform<my_tuple_element<std::decay_t<Tuples>>>
    >...
>;

template<class R, mp::int_... ituples, mp::int_... ivalues, class Tuple>
constexpr R my_tuple_cat_impl(
    emp::numbers<ituples...>, emp::numbers<ivalues...>, Tuple t)
{
    // get is looked up by argument-dependent lookup
    using std::get;
    return R{ get<ivalues>(get<ituples>(std::move(t)))... };
}

template<class... Tuples, class R = my_tuple_cat_result_type<Tuples...>>
constexpr R my_tuple_cat(Tuples&&... args)
{
    // ex:    tuple_size=3     tuple_size=2     tuple_size=4
    // list<    0, 0, 0,           1, 1,         2, 2, 2, 2   >
    using index_by_tuple = emp::make_int_sequence_c<
        sizeof...(Tuples),
        // repeat each index by number of element
        mp::each<mp::repeat<std::tuple_size<std::decay_t<Tuples>>>...,
            mp::join<>
        >
    >;

    // ex:    tuple_size=3     tuple_size=2     tuple_size=4
    // list<    0, 1, 2,           0, 1,         0, 1, 2, 3   >
    using index_by_value = emp::join<
        emp::make_int_sequence<std::tuple_size<std::decay_t<Tuples>>>...
    >;

    return my_tuple_cat_impl<R>(index_by_tuple{}, index_by_value{},
        std::tuple<Tuples&&...>(std::forward<Tuples>(args)...));
}

// defines a tuple like
//@{
namespace toy
{
    // tuple like
    struct Vector2D
    {
        int x, y;
    };

    template<std::size_t i>
    constexpr int get(Vector2D const& t)
    {
        return i == 0 ? t.x : t.y;
    }
}

template<>
struct std::tuple_size<::toy::Vector2D>
: std::integral_constant<std::size_t, 2>
{};

template<size_t i>
struct std::tuple_element<i, ::toy::Vector2D>
{
    using type = int;
};
//@}

// test
// @{
constexpr std::tuple<int, float, double> t0{1, 2, 3};
constexpr std::tuple<char, unsigned> t1{4, 5};
constexpr std::tuple<long> t2{6};
constexpr std::array<short, 4> a{7, 8, 9, 10};
constexpr toy::Vector2D v {11, 12};

constexpr auto my_tuple = my_tuple_cat(t0, t1, t2, a, v);

using my_tuple_type = std::remove_const_t<decltype(my_tuple)>;
using std_tuple = std::tuple<
    int, float, double,
    char, unsigned,
    long,
    short, short, short, short,
    int, int>;

static_assert(std::is_same_v<my_tuple_type, std_tuple>);
static_assert(my_tuple == std::tuple{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12});
// @}

FAQ

Functions are missing in the stacktrace when the compiler displays an error message, how to display them?

Compile with the define JLN_MP_ENABLE_DEBUG at 1 to have errors with more context.

Error: sorry, unimplemented: mangling record_type or sorry, unimplemented: mangling typename_type with Gcc.

This is a Gcc bug when an algorithm is used in the prototype of a function.

template<class... Ts>
mp::call<func, Ts...> foo();

// Must be replaced by

template<class... Ts, class R = mp::call<func, Ts...>>
R foo();