std::ranges::reserve_hint

 

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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

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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)

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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)

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view_interface	subrange

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

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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 as_input_viewviews::as_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

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

 

Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ { inline constexpr auto reserve_hint = /* unspecified */; }		(since C++26) (customization point object)
Call signature
template< class T > requires /* see below */ constexpr auto reserve_hint( T&& t );		(since C++26)

Estimates the number of elements in t in constant time.

Given the subexpression of which t denotes the (possibly materialized) result object as E, and the type of E as T:

• If ranges::size(E) is a valid expression, ranges::reserve_hint(E) is expression-equivalent to ranges::size(E).
• Otherwise, if auto(t.reserve_hint()) is a valid expression of integer-like type, ranges::reserve_hint(E) is expression-equivalent to auto(t.reserve_hint()).
• Otherwise, if all following conditions are satisfied, then ranges::reserve_hint(E) is expression-equivalent to auto(reserve_hint(t)):
  • T is a class or enumeration type.
  • auto(reserve_hint(t)) is a valid expression of integer-like type, where the meaning of reserve_hint is established as if by performing argument-dependent lookup only.
• Otherwise, ranges::reserve_hint(E) is ill-formed.

Diagnosable ill-formed cases above result in substitution failure when ranges::reserve_hint(E) appears in the immediate context of a template instantiation.

Customization point objects

The name ranges::reserve_hint denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Notes

Whenever ranges::reserve_hint(e) is valid for an expression e, the return type is integer-like.

Feature-test macro	Value	Std	Feature
__cpp_lib_ranges_reserve_hint	202502L	(C++26)	ranges::reserve_hint

See also

ranges::size (C++20)	returns an integer equal to the size of a range (customization point object)
ranges::ssize (C++20)	returns a signed integer equal to the size of a range (customization point object)
ranges::approximately_sized_range (C++26)	specifies that a range can estimate its size in constant time (concept)