std::ranges::constant_range

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Numerics library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	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 objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
movable-box	non-propagating-cache	simple-view

 

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)	(since C++23)

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

2) The exposition-only 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 <vector>
#include <string_view>
#include <span>
 
// mechanisms for ensuring the parameter is a constant range
// 1) an overload set where the mutable one defers to the constant one
template <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 parameter
template <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 itself
template <std::ranges::range R>
void takes_any_range3(R&& r) {
    if constexpr (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)));
    }
}
 
int main()
{
    static_assert(not std::ranges::constant_range<std::vector<int>>
              and std::ranges::constant_range<const std::vector<int>>
              and std::ranges::constant_range<std::string_view>
              and not std::ranges::constant_range<std::span<int>>
              and not std::ranges::constant_range<const std::span<int>>
              and std::ranges::constant_range<std::span<const int>>);
}