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std::ranges::views::concat, std::ranges::concat_view

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< cpp‎ | ranges
 
 
Ranges library
Range access
Range conversions
(C++23)

Range primitives



Dangling iterator handling
Range concepts
Views

Range factories
Range adaptors
Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
(until C++23)(C++23)


 
 
Defined in header <ranges>
template< ranges::input_range... Views >

    requires (ranges::view<Views> && ...) && (sizeof...(Views) > 0) &&
              /*concatable*/<Views...>
class concat_view

    : public ranges::view_interface<concat_view<Views...>>
(1) (since C++26)
namespace views {

    inline constexpr /* unspecified */ concat = /* unspecified */;

}
(2) (since C++26)
Call signature
template< ranges::input_range... Views >
constexpr ranges::view auto ranges::concat_view( Views&&... vs );
(since C++26)
template< ranges::input_range... Views >
constexpr /* range adaptor closure */ views::concat( Views&&... vs );
(since C++26)
Helper type aliases
template< class... Rs >

using /*concat-reference-t*/ =

    ranges::common_reference_t<ranges::range_reference_t<Rs>...>;
(3) (exposition only*)
template< class... Rs >
using /*concat-value-t*/ = std::common_type_t<ranges::range_value_t<Rs>...>;
(4) (exposition only*)
template< class... Rs >

using /*concat-rvalue-reference-t*/ =

    ranges::common_reference_t<ranges::range_rvalue_reference_t<Rs>...>;
(5) (exposition only*)
Helper concepts
template< class Ref, class RRef, class It >
concept /*concat-indirectly-readable-impl*/ = /* see description */;
(6) (exposition only*)
template< class... Rs >
concept /*concatable*/ = /* see description */;
(7) (exposition only*)
template< bool Const, class... Rs >
concept /*concat-is-random-access*/ = /* see description */;
(8) (exposition only*)
template< bool Const, class... Rs >
concept /*concat-is-bidirectional*/ = /* see description */;
(9) (exposition only*)

concat_view presents a view factory that takes an arbitrary number of ranges as an argument list, and provides a view that starts at the first element of the first range, ends at the last element of the last range, with all range elements sequenced in between respectively in the order given in the arguments, effectively concatenating, or chaining together the argument ranges.

1) The class template with a template parameter that is a non-empty pack of views each of which models at least input_range and is concatable (7).
2) The name views::concat denotes a RangeAdaptorObject. Given subexpressions p and q, the expression views::concat(p, q) is expression-equivalent to ranges::concat_view(p, q).
3) Represents the reference type. An extra constraint is needed to make sure that each underlying range’s ranges::range_reference_t is convertible to ranges::common_reference_t.
4) The iterator::value_type that additionally respects the underlying ranges’ value_type to support the cases when the underlying ranges have proxy iterators.
5) The rvalue reference that also correctly supports the cases where underlying iterators customize iter_move.
6) Defines the indirectly-readable concept for the iterator so that concat_view can model input_range. Equivalent to:
template< class... Rs >
concept /*concat-indirectly-readable*/ = // exposition only
    std::common_reference_with</*concat-reference-t*/<Rs...>&&,
                               /*concat-value-t*/<Rs...>&> &&
    std::common_reference_with</*concat-reference-t*/<Rs...>&&,
                               /*concat-rvalue-reference-t*/<Rs...>&&> &&
    std::common_reference_with</*concat-rvalue-reference-t*/<Rs...>&&,
                               /*concat-value-t*/<Rs...> const&> &&
    (/*concat-indirectly-readable-impl*/</*concat-reference-t*/<Rs...>,
                                         /*concat-rvalue-reference-t*/<Rs...>,
                                         ranges::iterator_t<Rs>> && ...);

where exposition-only concept /*concat-indirectly-readable-impl*/ is

template< class Ref, class RRef, class It >
concept /*concat-indirectly-readable-impl*/ = // exposition only
    requires(const It it) {
        { *it } -> std::convertible_to<Ref>;
        { ranges::iter_move(it)} -> std::convertible_to<RRef>;
    };
7) Determines whether any two or more different ranges can be adapted into a sequence that itself models a range. Equivalent to:
template< class... Rs >
concept /*concatable*/ = requires { // exposition only
        typename /*concat-reference-t*/<Rs...>;
        typename /*concat-value-t*/<Rs...>;
        typename /*concat-rvalue-reference-t*/<Rs...>;
    } && /*concat-indirectly-readable*/<Rs...>;
8) Let Fs be the pack that consists of all elements of Rs except the last element. Equivalent to:
template<bool Const, class... Rs>
concept /*concat-is-random-access*/ = // exposition only
    /*all-random-access*/<Const, Rs...> &&
    (common_range</*maybe-const*/<Const, Fs>> && ...);
9) Let Fs be the pack that consists of all elements of Rs except the last element. Equivalent to:
template<bool Const, class... Rs>
concept /*concat-is-bidirectional*/ = // exposition only
    /*all-bidirectional*/<Const, Rs...> &&
    (common_range</*maybe-const*/<Const, Fs>> && ...);

concat_view always models input_range, and models forward_range, bidirectional_range, random_access_range, or sized_range if each adapted view type models the corresponding concept.

concat_view can be common_range if the last underlying range models common_range.

Contents

Customization point objects

The name views::concat denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __concat_fn.

All instances of __concat_fn are equal. The effects of invoking different instances of type __concat_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, views::concat can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to views::concat above, __concat_fn models

Otherwise, no function call operator of __concat_fn participates in overload resolution.

[edit] Member objects

Name Definition
views_ (private) A tuple of type equivalent to std::tuple<Views...>, that holds all adapted view objects.
(exposition-only member object*)

[edit] Member functions

constructs a concat_view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function) [edit]
Inherited from std::ranges::view_interface
returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>) [edit]
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>) [edit]
returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Deduction guides

[edit] Nested classes

Class name Definition
the iterator type
(exposition-only member class template*)

[edit] Helper templates

There is no specialization of ranges::enable_borrowed_range for concat_view, because this would require the iterator implementation to contain a copy of all iterators and sentinels of all underlying ranges at all times.

[edit] Notes

No argument views::concat() is ill-formed, because there is no reasonable way to determine an element type T. Single argument views::concat(r) is expression equivalent to views::all(r).

Feature-test macro Value Std Feature
__cpp_lib_ranges_concat 202403L (C++26) std::ranges::concat_view

[edit] Example

The preliminary version can be checked out on Compiler Explorer.

#include <cassert>
#include <list>
#include <print>
#include <ranges>
#include <vector>
 
int main()
{
    std::vector<int> v0{1, 2, 3}, v1{4, 5};
    int a[]{6, 7};
    int i{8};
    auto ie{std::views::single(i)};
 
    auto con = std::views::concat(v0, v1, a, ie);
    assert(con.size() == v0.size() + v1.size() + std::size(a) + ie.size());
    std::println("con.size(): {}", con.size());
    std::println("con: {}", con);
    con[6] = 42; // con is random_access_range, operator[] returns a reference
    assert(a[1] == 42); // a[1] was modified via con[6]
    std::println("con: {}", con);
 
    std::list<int> l{7, 8}; // list is bidirectional range
    auto cat = std::views::concat(v0, l);
    std::println("cat: {}", cat);
    // cat[0] = 13; // compile-time error: cat is bidirectional => no operator[]
}

Output:

con.size(): 8
con: [1, 2, 3, 4, 5, 6, 7, 8]
con: [1, 2, 3, 4, 5, 6, 42, 8]
cat: [1, 2, 3, 7, 8]

[edit] References

  • C++26 standard (ISO/IEC 14882:2026):
  • 26.7.18 Concat view [range.concat]

[edit] See also

a view consisting of the sequence obtained from flattening a view of ranges
(class template) (range adaptor object)[edit]
a view consisting of the sequence obtained from flattening a view of ranges, with the delimiter in between elements
(class template) (range adaptor object)[edit]
a view consisting of tuples of references to corresponding elements of the adapted views
(class template) (customization point object)[edit]
a view consisting of tuples of results calculated by the n-ary cartesian product of the adapted views
(class template) (customization point object)[edit]