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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../ranges/reserve_hint.html below:

std::ranges::reserve_hint - cppreference.com

Call signature

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.

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.

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

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