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_borrowed_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 object 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_reference_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 potentially-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 different 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 interchangeably.
   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 failure.
   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 implementations.

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 contiguous 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)