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 Description

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

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 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

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.

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

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 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

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 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

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 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

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 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

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.

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

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 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

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 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

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 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

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 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()

template<ptrdiff_t idx, typename head_t, typename... tail_t>

auto seqan3::pack_traits::detail::at

(

)

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()

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 Parameters

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

find()

template<typename query_t, typename... pack_t>

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

constexpr

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()

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

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

constexpr

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()

template<typename head_t, typename... tail_t>

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

(

)

Implementation for seqan3::pack_traits::front.

Template Parameters

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

replace_at()

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 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()

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.

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

template<typename query_t, typename... pack_t>

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

inlineconstexpr

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

template<typename query_t, typename... pack_t>

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

inlineconstexpr

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

template<typename query_t, typename... pack_t>

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

inlineconstexpr

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

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

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

inlineconstexpr

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

template<typename... pack_t>

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

inlineconstexpr

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);
}