std::ranges::size(3) C++ Standard Libary std::ranges::size(3) NAME std::ranges::size - std::ranges::size Synopsis Defined in header <ranges> Defined in header <iterator> inline namespace /* unspecified */ { (since C++20) inline constexpr auto size = /* unspecified */; (customization point object) } Call signature template< class T > requires /* see below */ (since C++20) constexpr auto size( T&& t ); Calculates the number of elements in t in constant time. A call to ranges::size is expression-equivalent to: 1. decay-copy(std::extent_v<T>) (until C++23) auto(std::extent_v<T>) (since C++23), if T is an array type with a known bound. 2. Otherwise, decay-copy(t.size()) (until C++23) auto(t.size()) (since C++23), if ranges::disable_sized_range<std::remove_cv_t<T>> is false, and that expression is valid and has an integer-like type. 3. Otherwise, decay-copy(size(t)) (until C++23) auto(size(t)) (since C++23), if ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted expression is valid and has an integer-like type, where the meaning of size is established as if by performing argument-depen- dent lookup only. 4. Otherwise, /* to-unsigned-like */(ranges::end(t) - ranges::be- gin(t)), if T models ranges::forward_range and ranges::sentinel_t<T> models std::sized_sentinel_for<ranges::iterator_t<T>>, where /* to-unsigned-like */ denotes an explicit conversion to an unsigned-integer-like type. In all other cases, a call to ranges::size is ill-formed, which can re- sult in substitution failure when ranges::size(t) appears in the immediate con- text of a template instantiation. Notes Whenever ranges::size(e) is valid for an expression e, the return type is integer-like. The C++20 standard requires that if the underlying size function call returns a prvalue, the return value is move-constructed from the materialized tem- porary object. All implementations directly return the prvalue instead. The re- quirement is corrected by the post-C++20 proposal P0849R8 to match the implementa- tions. The expression ranges::distance(e) can also be used to determine the size of a range e. Unlike ranges::size(e), ranges::distance(e) works even if e is an un- sized range, at the cost of having linear complexity in that case. Example // Run this code #include <iostream> #include <ranges> #include <type_traits> #include <vector> int main() { auto v = std::vector<int>{}; std::cout << "ranges::size(v) == " << std::ranges::size(v) << '\n'; auto il = {7}; // std::initializer_list std::cout << "ranges::size(il) == " << std::ranges::size(il) << '\n'; int array[]{4, 5}; // array has a known bound std::cout << "ranges::size(array) == " << std::ranges::size(array) << '\n'; static_assert(std::is_signed_v<decltype(std::ranges::size(v))> == false); } Output: ranges::size(v) == 0 ranges::size(il) == 1 ranges::size(array) == 2 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P2602R2 C++20 there's machinery to prohibit certain removed such machinery non-member size found by ADL See also ranges::ssize returns a signed integer equal to the size of a range (C++20) (customization point object) ranges::sized_range specifies that a range knows its size in constant time (C++20) (concept) ranges::distance returns the distance between an iterator and a sen- tinel, or (C++20) between the beginning and end of a range (niebloid) size ssize returns the size of a container or array (C++17) (function template) (C++20) http://cppreference.com 2024.06.10 std::ranges::size(3)
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