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

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)

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

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

Estimates the number of elements int in constant time.

Given thesubexpression of whicht denotes the (possiblymaterialized) result object asE, and the type ofE asT:

Ifranges::size(E) is a valid expression,ranges::reserve_hint(E) isexpression-equivalent toranges::size(E).
Otherwise, ifauto(t.reserve_hint()) is a valid expression ofinteger-like type,ranges::reserve_hint(E) is expression-equivalent toauto(t.reserve_hint()).
Otherwise, if all following conditions are satisfied, thenranges::reserve_hint(E) is expression-equivalent toauto(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 ofreserve_hint is established as if by performingargument-dependent lookup only.
Otherwise,ranges::reserve_hint(E) is ill-formed.

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

Customization point objects

The nameranges::reserve_hint denotes acustomization point object, which is a constfunction object of aliteral semiregular class type. SeeCustomizationPointObject for details.

[edit]Notes

Wheneverranges::reserve_hint(e) is valid for an expressione, the return type isinteger-like.

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

[edit]Example

This section is incomplete
Reason: no example

[edit]See also

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