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std::ranges::size(3) C++ Standard Libary std::ranges::size(3) NAME std::ranges::size - std::ranges::size Synopsis Defined in header <ranges> 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. Let t be an object of type T. A call to ranges::size is expression- equivalent to: 1. std::extent_v<T>, if T is an array type with a known bound. 2. Otherwise, t.size() converted to its decayed type, if ranges::disable_sized_range<std::remove_cv_t<T>> is false, and the converted expression is valid and has an integer-like type. 3. Otherwise, size(t) converted to its decayed type, 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 overload resolution is performed with the following candidates: * void size(auto&) = delete; * void size(const auto&) = delete; 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 con- version to an unsigned-integer-like type. In all other cases, a call to ranges::size is ill-formed, which can result in substitution failure when ranges::size(t) appears in the immediate context of a template instantiation. Expression-equivalent Expression e is expression-equivalent to expression f, if * e and f have the same effects, and * either both are constant subexpressions or else neither is a constant subexpression, and * either both are potentially-throwing or else neither is poten- tially-throwing (i.e. noexcept(e) == noexcept(f)). Customization point objects The name ranges::size denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __size_fn. All instances of __size_fn are equal. The effects of invoking dif- ferent instances of type __size_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::size can be copied freely and its copies can be used inter- changeably. Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::size above, __size_fn models * std::invocable<__size_fn, Args...>, * std::invocable<const __size_fn, Args...>, * std::invocable<__size_fn&, Args...>, and * std::invocable<const __size_fn&, Args...>. Otherwise, no function call operator of __size_fn participates in overload resolution. 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 temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8 to match the implemen- tations. 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::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'; std::cout << std::boolalpha << "is_signed: " << std::is_signed_v<decltype(std::ranges::size(v))> << '\n'; } Output: ranges::size(v) == 0 ranges::size(il) == 1 ranges::size(array) == 2 is_signed: false 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 con- stant time (C++20) (concept) size ssize returns the size of a container or array (C++17) (function template) (C++20) http://cppreference.com 2022.07.31 std::ranges::size(3)
NAME | Synopsis | Notes | Example | Output: | See also
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