std::ranges::constant_range

 

C++

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Feature test macros(C++20)

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Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

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Date and time library

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Filesystem library(C++17)

Concurrency support library(C++11)

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Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted to_input_viewviews::to_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
template<class T > concept constant_range =     ranges::input_range<T>&&     /*constant-iterator*/<ranges::iterator_t<T>>;	(1)	(since C++23)
Helper concepts
template<class T > concept /*constant-iterator*/=     std::input_iterator<T>&&     std::same_as<std::iter_const_reference_t<T>, std::iter_reference_t<T>>;	(2)	(exposition only*)

1) The constant_range concept is a refinement of range for which ranges::begin returns a constant iterator.

2) The concept /*constant-iterator*/<T> is satisfied when the result of the indirection operation of the input iterator is its const reference type which implies read-only.

Example

Run this code

#include <ranges>#include <span>#include <string_view>#include <vector>   // mechanisms for ensuring the parameter is a constant range// 1) an overload set where the mutable one defers to the constant onetemplate<std::ranges::constant_range R>void takes_any_range1(R&& r){// R is definitely a constant range}   template<std::ranges::range R>void takes_any_range1(R&& r){ takes_any_range1(std::views::as_const(std::forward<R>(r)));}   // 2) one function template that shadows its parametertemplate<std::ranges::range R>void takes_any_range2(R&& _r){auto r = std::views::as_const(std::forward<R>(_r));   // r is definitely a constant range// never use _r again}   // 3) one function template that recursively invokes itselftemplate<std::ranges::range R>void takes_any_range3(R&& r){ifconstexpr(std::ranges::constant_range<R>){// R is definitely a constant range// put implementation here}else takes_any_range3(std::views::as_const(std::forward<R>(r)));}   static_assert ( std::ranges::constant_range<conststd::vector<int>> and not std::ranges::constant_range<std::vector<int>> and std::ranges::constant_range<std::string_view> and not std::ranges::constant_range<std::span<int>> and std::ranges::constant_range<std::span<constint>> and not std::ranges::constant_range<conststd::span<int>>);   int main(){}