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std::ranges::data(3) C++ Standard Libary std::ranges::data(3) NAME std::ranges::data - std::ranges::data Synopsis Defined in header <ranges> inline namespace /*unspecified*/ { (since C++20) inline constexpr /*unspecified*/ data = /*unspecified*/; (customization point object) } Call signature template< class T > requires /* see below */ (since C++20) constexpr std::remove_reference_t<ranges::range_reference_t<T>>* data( T&& t ); Returns a pointer to the first element of a contiguous range. If the argument is an lvalue or ranges::enable_bor- rowed_range<std::remove_cv_t<T>> is true, a call to ranges::data is expression-equivalent to: 1. std::forward<T>(t).data() converted to its decayed type, if that converted expression is valid, and its return type is a pointer to an ob- ject type. 2. Otherwise, std::to_address(ranges::begin(std::forward<T>(t))), if ranges::begin(std::forward<T>(t)) is valid and returns a type that models std::contiguous_iterator. If std::remove_all_extents_t<std::remove_refer- ence_t<T>> is incomplete, then ranges::data(std::forward<T>(t)) is ill-formed, no diagnostic required. In all other cases, a call to ranges::data is ill-formed, which can result in substitution failure when ranges::data(e) 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::data 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 __data_fn. All instances of __data_fn are equal. The effects of invoking dif- ferent instances of type __data_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::data 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::data above, __data_fn models * std::invocable<__data_fn, Args...>, * std::invocable<const __data_fn, Args...>, * std::invocable<__data_fn&, Args...>, and * std::invocable<const __data_fn&, Args...>. Otherwise, no function call operator of __data_fn participates in overload resolution. 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::data is ill-formed, which also results in substitution fail- ure. If ranges::data(e) is valid for an expression e, then it returns a pointer to an object. The C++20 standard requires that if the underlying data 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 <cstring> #include <iostream> #include <ranges> #include <string> int main() { std::string s {"Hello world!\n"}; char a[20]; // storage for a C-style string std::strcpy(a, std::ranges::data(s)); // [data(s), data(s) + size(s)] is guaranteed to be an NTBS std::cout << a; } Output: Hello world! See also ranges::cdata obtains a pointer to the beginning of a read-only con- tiguous range (C++20) (customization point object) ranges::begin returns an iterator to the beginning of a range (C++20) (customization point object) data obtains the pointer to the underlying array (C++17) (function template) http://cppreference.com 2022.07.31 std::ranges::data(3)
NAME | Synopsis | Notes | Example | Output: | See also
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