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std::ranges::begin(3) C++ Standard Libary std::ranges::begin(3) NAME std::ranges::begin - std::ranges::begin Synopsis Defined in header <ranges> Defined in header <iterator> inline namespace /* unspecified */ { inline constexpr /* unspecified */ begin = /* (since C++20) unspecified */; (customiza- tion point object) } Call signature template< class T > requires /* see below */ (since C++20) constexpr std::input_or_output_iterator auto begin( T&& t ); Returns an iterator to the first element of the argument. range-begin-end.svg If the argument is an lvalue or ranges::enable_bor- rowed_range<std::remove_cv_t<T>> is true, then a call to ranges::begin is expression-equivalent to: 1. t + 0 if t has an array type. * If std::remove_all_extents_t<std::remove_reference_t<T>> is incomplete, then the call to ranges::begin is ill-formed, no diagnostic required. 2. Otherwise, decay-copy(t.begin()) (until C++23) auto(t.begin()) (since C++23), if that expression is valid and its type models std::input_or_output_iterator. 3. Otherwise, decay-copy(begin(t)) (until C++23) auto(begin(t)) (since C++23), if T is a class or enumeration type, that expres- sion is valid and its type models std::input_or_output_iterator, where the meaning of begin is established as if by performing argument-dependent lookup only. In all other cases, a call to ranges::begin is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation. Notes If the argument is an rvalue (i.e. T is an object type) and ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to ranges::begin is ill-formed, which also results in substitution failure. The return type models std::input_or_output_iterator in all cases. The C++20 standard requires that if the underlying begin 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 <cassert> #include <ranges> #include <vector> int main() { std::vector v{3, 1, 4}; auto vi = std::ranges::begin(v); auto vci = std::ranges::cbegin(v); assert(*vi == 3 and *vi == *vci); ++vi; ++vci; // OK: vci is modifiable object *vi = 42; // OK: vi points to mutable element // *vci = 13; // Error: vci points to immutable element int a[]{-5, 10, 15}; auto ai = std::ranges::begin(a); // works with C-arrays as well assert(*ai == -5); *ai = 42; // OK } 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 re- moved such machinery non-member begin found by ADL See also ranges::cbegin returns an iterator to the beginning of a read-only range (C++20) (customization point object) begin cbegin returns an iterator to the beginning of a container or array (C++11) (function template) (C++14) http://cppreference.com 2024.06.10 std::ranges::begin(3)
NAME | Synopsis | Notes | Example | See also
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