Type Pack

Provides metaprogramming utilities for working on template parameter packs. More...

Collaboration diagram for Type Pack:

Functions
template<ptrdiff_t idx, typename head_t , typename... tail_t>
auto	seqan3::pack_traits::detail::at ()
	Implementation for seqan3::pack_traits::at.
template<typename head_t , typename... tail_t>
type_list< tail_t... >	seqan3::pack_traits::detail::drop_front ()
	Implementation for seqan3::pack_traits::drop_front.
template<typename query_t , typename... pack_t>
constexpr ptrdiff_t	seqan3::pack_traits::detail::find ()
	Implementation for seqan3::pack_traits::find.
template<template< typename > typename pred_t, typename... pack_t>
constexpr ptrdiff_t	seqan3::pack_traits::detail::find_if ()
	Implementation for seqan3::pack_traits::find_if.
template<typename head_t , typename... tail_t>
std::type_identity< head_t >	seqan3::pack_traits::detail::front ()
	Implementation for seqan3::pack_traits::front.
template<typename replace_t , ptrdiff_t idx, typename... pack_t, size_t... i>
auto	seqan3::pack_traits::detail::replace_at (std::index_sequence< i... >) -> type_list< std::conditional_t< i==idx, replace_t, pack_t >... >
	Implementation for seqan3::pack_traits::replace_at.
template<ptrdiff_t idx, typename head_t , typename... pack2_t, typename... pack1_t>
auto	seqan3::pack_traits::detail::split_after (type_list< pack1_t... >)
	Implementation for seqan3::pack_traits::split_after.

Type pack traits (return a single type)
template<ptrdiff_t idx, typename... pack_t>
using	seqan3::pack_traits::at = typename decltype(detail::at< idx, pack_t... >())::type
	Return the type at given index from the type pack.
template<typename... pack_t>
using	seqan3::pack_traits::front = typename decltype(detail::front< pack_t... >())::type
	Return the first type from the type pack.
template<typename... pack_t>
using	seqan3::pack_traits::back = typename decltype((std::type_identity< pack_t >{},...))::type
	Return the last type from the type pack.

Type pack traits (return a type list)
template<typename... pack_t>
using	seqan3::pack_traits::drop_front = typename decltype(detail::drop_front< pack_t... >())::type
	Return a seqan3::type_list of all the types in the type pack, except the first.
template<template< typename > typename trait_t, typename... pack_t>
using	seqan3::pack_traits::transform = seqan3::type_list< trait_t< pack_t >... >
	Apply a transformation trait to every type in the pack and return a seqan3::type_list of the results.
template<ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::take = typename decltype(detail::split_after< i, pack_t... >(type_list<>{}))::first_type
	Return a seqan3::type_list of the first n types in the type pack.
template<ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::drop = typename decltype(detail::split_after< i, pack_t... >(type_list<>{}))::second_type
	Return a seqan3::type_list of the types in the type pack, except the first n.
template<ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::take_last = drop< size< pack_t... > - i, pack_t... >
	Return a seqan3::type_list of the last n types in the type pack.
template<ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::drop_last = take< size< pack_t... > - i, pack_t... >
	Return a seqan3::type_list of the types the type pack, except the last n.
template<ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::split_after = decltype(detail::split_after< i, pack_t... >(type_list<>{}))
	Split a type pack into two parts returned as a pair of seqan3::type_list.
template<typename replace_t , std::ptrdiff_t i, typename... pack_t>
using	seqan3::pack_traits::replace_at = decltype(detail::replace_at< replace_t, i, pack_t... >(std::make_index_sequence< size< pack_t... > >{}))
	Replace the type at the given index with the given type.

Type pack traits (return a value)
template<typename... pack_t>
constexpr size_t	seqan3::pack_traits::size = sizeof...(pack_t)
	The size of a type pack.
template<typename query_t , typename... pack_t>
constexpr ptrdiff_t	seqan3::pack_traits::count = (std::is_same_v<query_t, pack_t> + ... + 0)
	Count the occurrences of a type in a pack.
template<typename query_t , typename... pack_t>
constexpr ptrdiff_t	seqan3::pack_traits::find = seqan3::pack_traits::detail::find<query_t, pack_t...>()
	Get the index of the first occurrence of a type in a pack.
template<template< typename > typename pred_t, typename... pack_t>
constexpr ptrdiff_t	seqan3::pack_traits::find_if = seqan3::pack_traits::detail::find_if<pred_t, pack_t...>()
	Get the index of the first type in a pack that satisfies the given predicate.
template<typename query_t , typename... pack_t>
constexpr bool	seqan3::pack_traits::contains = (find<query_t, pack_t...> != -1)
	Whether a type occurs in a pack or not.

Detailed Descriptionno-api

Provides metaprogramming utilities for working on template parameter packs.

See also

Utility

This module contains auxiliary functions, algorithms and traits on template parameter packs. All traits on a template parameter pack are defined in the header <seqan3/utility/type_pack/traits.hpp>.

Typedef Documentation

at [no-api]

using seqan3::pack_traits::at = typedef typename decltype(detail::at<idx, pack_t...>())::type

no-api

Return the type at given index from the type pack.

Template Parameters

idx	The index; must be smaller than the size of the pack.
pack_t	The type pack.

Negative indexes are supported (e.g. at<-1, int, double, bool &> is bool &).

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Look at the 2nd element.
    static_assert(std::same_as<float, seqan3::pack_traits::at<1, int, float, bool, double>>);
    // Look at the last element.
    static_assert(std::same_as<double, seqan3::pack_traits::at<-1, int, float, bool, double>>);
}

back [no-api]

using seqan3::pack_traits::back = typedef typename decltype((std::type_identity<pack_t>{}, ...))::type

no-api

Return the last type from the type pack.

Template Parameters

pack_t

The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n) (possibly O(1))
• Other operations: O(1)

Notably faster than seqan3::pack_traits::at<size<pack...> - 1, pack...> (no recursive template instantiations).

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Check if the last value is float.
    static_assert(std::same_as<float, seqan3::pack_traits::back<int, float, bool, int, float>>);
}

drop [no-api]

using seqan3::pack_traits::drop = typedef typename decltype(detail::split_after<i, pack_t...>(type_list<>{}))::second_type

no-api

Return a seqan3::type_list of the types in the type pack, except the first n.

Template Parameters

i	The amount to drop; must be >= 0 and <= the size of the type pack.
pack_t	The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Drop the first two types in the pack.
    static_assert(std::same_as<seqan3::type_list<bool, int>, seqan3::pack_traits::drop<2, float, double, bool, int>>);
}

drop_front [no-api]

using seqan3::pack_traits::drop_front = typedef typename decltype(detail::drop_front<pack_t...>())::type

no-api

Return a seqan3::type_list of all the types in the type pack, except the first.

Template Parameters

pack_t

The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(1)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Return the a type list of the pack without the first type.
    static_assert(
        std::same_as<seqan3::type_list<float, bool, int>, seqan3::pack_traits::drop_front<int, float, bool, int>>);
}

drop_last [no-api]

using seqan3::pack_traits::drop_last = typedef take<size<pack_t...> - i, pack_t...>

no-api

Return a seqan3::type_list of the types the type pack, except the last n.

Template Parameters

i	The amount to drop; must be >= 0 and <= the size of the type pack.
pack_t	The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Take the last two types in the pack.
    static_assert(std::same_as<seqan3::type_list<bool, int>, seqan3::pack_traits::take_last<2, int, float, bool, int>>);
}

front [no-api]

using seqan3::pack_traits::front = typedef typename decltype(detail::front<pack_t...>())::type

no-api

Return the first type from the type pack.

Template Parameters

pack_t

The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(1)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Check if the first value is int.
    static_assert(std::same_as<int, seqan3::pack_traits::front<int, float, bool, int, float>>);
}

replace_at [no-api]

using seqan3::pack_traits::replace_at = typedef decltype(detail::replace_at<replace_t, i, pack_t...>(std::make_index_sequence<size<pack_t...> >{}))

no-api

Replace the type at the given index with the given type.

Template Parameters

replace_t	The type to replace the old type with.
i	The index of the type to be replaced.
pack_t	The (input) type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Take the last two types in the pack.
    static_assert(std::same_as<seqan3::type_list<bool, int>, seqan3::pack_traits::take_last<2, int, float, bool, int>>);
}

split_after [no-api]

using seqan3::pack_traits::split_after = typedef decltype(detail::split_after<i, pack_t...>(type_list<>{}))

no-api

Split a type pack into two parts returned as a pair of seqan3::type_list.

Template Parameters

i	The number of elements after which to split; must be >= 0 and <= the size of the type pack.
pack_t	The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Take the last two types in the pack.
    static_assert(std::same_as<seqan3::type_list<bool, int>, seqan3::pack_traits::take_last<2, int, float, bool, int>>);
}

take [no-api]

using seqan3::pack_traits::take = typedef typename decltype(detail::split_after<i, pack_t...>(type_list<>{}))::first_type

no-api

Return a seqan3::type_list of the first n types in the type pack.

Template Parameters

i	The target size; must be >= 0 and <= the size of the type pack.
pack_t	The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
 
    // Take the first two types in the pack.
    static_assert(std::same_as<seqan3::type_list<int, float>, seqan3::pack_traits::take<2, int, float, bool, double>>);
}

take_last [no-api]

using seqan3::pack_traits::take_last = typedef drop<size<pack_t...> - i, pack_t...>

no-api

Return a seqan3::type_list of the last n types in the type pack.

Template Parameters

i	The target size; must be >= 0 and <= the size of the type pack.
pack_t	The type pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Take the last two types in the pack.
    static_assert(std::same_as<seqan3::type_list<bool, int>, seqan3::pack_traits::take_last<2, int, float, bool, int>>);
}

transform [no-api]

using seqan3::pack_traits::transform = typedef seqan3::type_list<trait_t<pack_t>...>

no-api

Apply a transformation trait to every type in the pack and return a seqan3::type_list of the results.

Template Parameters

trait_t	The transformation trait, can be an alias template, e.g. a transformation trait shortcut.
pack_t	The type pack.

The transformation trait given as first argument can be an alias template, e.g. std::type_identity_t, not std::type_identity. The alias must take exactly one argument and be defined for all types in the pack.

(Compile-time) Complexity

• Number of template instantiations: O(n)
• Other operations: O(1)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <list>
#include <vector>
 
#include <seqan3/core/range/type_traits.hpp>
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Transform the types in the pack into reference types.
    static_assert(std::same_as<
                  seqan3::pack_traits::
                      transform<std::ranges::range_reference_t, std::vector<int>, std::vector<float>, std::list<bool>>,
                  seqan3::type_list<int &, float &, bool &>>);
}

Function Documentation

at() [no-api]

auto seqan3::pack_traits::detail::at ( )

no-api

Implementation for seqan3::pack_traits::at.

Template Parameters

idx	The index.
head_t	Currently viewed pack_t element.
tail_t	Rest of the type pack.

drop_front() [no-api]

type_list< tail_t... > seqan3::pack_traits::detail::drop_front ( )

no-api

Implementation for seqan3::pack_traits::drop_front.

Template Parameters

head_t	Currently viewed pack_t element.
tail_t	Rest of the type pack.

find() [no-api]

template<typename query_t , typename... pack_t>

constexpr ptrdiff_t seqan3::pack_traits::detail::find ( )

no-apiconstexpr

Implementation for seqan3::pack_traits::find.

Template Parameters

query_t	The type you are searching for.
pack_t	The type pack.

Returns

The position of the first occurrence of query_t in pack_t or -1 if it is not contained.

find_if() [no-api]

template<template< typename > typename pred_t, typename... pack_t>

constexpr ptrdiff_t seqan3::pack_traits::detail::find_if ( )

no-apiconstexpr

Implementation for seqan3::pack_traits::find_if.

Template Parameters

pred_t	The predicate that is being evaluated.
pack_t	The type pack.

Returns

The position of the first type t in pack_t for whom pred_t<t>::value is true.

front() [no-api]

std::type_identity< head_t > seqan3::pack_traits::detail::front ( )

no-api

Implementation for seqan3::pack_traits::front.

Template Parameters

head_t	Currently viewed pack_t element.
tail_t	Rest of the type pack.

replace_at() [no-api]

auto seqan3::pack_traits::detail::replace_at ( std::index_sequence< i... >  ) -> type_list< std::conditional_t< i==idx, replace_t, pack_t >... >

no-api

Implementation for seqan3::pack_traits::replace_at.

Template Parameters

replace_t	The type replacing the old one.
idx	The index of the type to replace.
pack_t	The type pack to be modified.
i	The indicies of the index sequence associated with the type pack.

split_after() [no-api]

auto seqan3::pack_traits::detail::split_after ( type_list< pack1_t... >  )

no-api

Implementation for seqan3::pack_traits::split_after.

Template Parameters

idx	The index to split the type pack at.
pack1_t	The type pack before the split index.
head_t	The next type that is moved before split index.
pack2_t	The type pack after the split index.

Variable Documentation

contains [no-api]

template<typename query_t , typename... pack_t>

constexpr bool seqan3::pack_traits::contains = (find<query_t, pack_t...> != -1)

no-apiinlineconstexpr

Whether a type occurs in a pack or not.

Template Parameters

query_t	The type you are searching for.
pack_t	The type pack.

Returns

true or false.

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(n), possibly == i, where i is the index of the first occurrence

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Double is not in the pack so find returns -1. However, bool is in the pack so find will return 2.
    static_assert(seqan3::pack_traits::find<double, int, float, bool> == -1);
    static_assert(seqan3::pack_traits::find<bool, int, float, bool> == 2);
}

count [no-api]

template<typename query_t , typename... pack_t>

constexpr ptrdiff_t seqan3::pack_traits::count = (std::is_same_v<query_t, pack_t> + ... + 0)

no-apiinlineconstexpr

Count the occurrences of a type in a pack.

Template Parameters

query_t	The type you are searching for.
pack_t	The type pack.

Returns

The number of occurrences of the query_t in pack_t.

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(n)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Count the number of type int in the pack.
    static_assert(seqan3::pack_traits::count<int, int, float, bool, int> == 2);
}

find [no-api]

template<typename query_t , typename... pack_t>

constexpr ptrdiff_t seqan3::pack_traits::find = seqan3::pack_traits::detail::find<query_t, pack_t...>()

no-apiinlineconstexpr

Get the index of the first occurrence of a type in a pack.

Template Parameters

query_t	The type you are searching for.
pack_t	The type pack.

Returns

The position of the first occurrence of query_t in pack_t or -1 if it is not contained.

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(n), possibly == i, where i is the return value

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Double is not in the pack so find returns -1. However, bool is in the pack so find will return 2.
    static_assert(seqan3::pack_traits::find<double, int, float, bool> == -1);
    static_assert(seqan3::pack_traits::find<bool, int, float, bool> == 2);
}

find_if [no-api]

template<template< typename > typename pred_t, typename... pack_t>

constexpr ptrdiff_t seqan3::pack_traits::find_if = seqan3::pack_traits::detail::find_if<pred_t, pack_t...>()

no-apiinlineconstexpr

Get the index of the first type in a pack that satisfies the given predicate.

Template Parameters

pred_t	The predicate that is being evaluated (a class template).
pack_t	The type pack.

Returns

The index or -1 if no types match.

Note that the predicate must be given as a type template (variable templates cannot be passed as template arguments unfortunately). This means e.g. find_if<std::is_integral, float, double, int, float> (not std::is_integral_v!).

(Compile-time) Complexity

• Number of template instantiations: O(n), possibly == i, where i is the return value

• Other operations: O(n), possibly == i, where i is the return value

  Only the predicate is instantiated.

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // None of the types in t is a pointer so find_if returns -1. However, int and bool are both integral,
    // so find_if returns 0 for the first occurrence.
    static_assert(seqan3::pack_traits::find_if<std::is_pointer, int, float, double> == -1);
    static_assert(seqan3::pack_traits::find_if<std::is_integral, int, float, double> == 0);
}

size [no-api]

template<typename... pack_t>

constexpr size_t seqan3::pack_traits::size = sizeof...(pack_t)

no-apiinlineconstexpr

The size of a type pack.

Template Parameters

pack_t

The type pack.

Returns

sizeof...(pack_t)

(Compile-time) Complexity

• Number of template instantiations: O(1)
• Other operations: O(1)

// SPDX-FileCopyrightText: 2006-2025 Knut Reinert & Freie Universität Berlin
// SPDX-FileCopyrightText: 2016-2025 Knut Reinert & MPI für molekulare Genetik
// SPDX-License-Identifier: CC0-1.0
 
#include <seqan3/utility/type_pack/traits.hpp>
 
int main()
{
    // Get the size of the pack.
    static_assert(seqan3::pack_traits::size<int, float, bool, int> == 4);
}