std::ranges::cdata(3)

NAME
std::ranges::cdata - std::ranges::cdata

Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   inline namespace /*unspecified*/ {
       inline constexpr /*unspecified*/ cdata = (since C++20)
   /*unspecified*/; (customization point object)
   }
   Call signature
   template< class T >
       requires /* see below */ (since C++20)
   constexpr /* see below */ cdata( T&& t );
   Returns a pointer to the first element
   of constant type
   (since C++23) of a contiguous range denoted by a
   const-qualified
   (until C++23) argument.
   Let CT be
     * const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is
       an lvalue reference type),
     * const T otherwise. (until
                        C++23)
   A call to ranges::cdata is expression-equivalent to
   ranges::data(static_cast<CT&&>(t)).
   The return type is equivalent to
   std::remove_reference_t<ranges::range_reference_t<CT>>*.
   If the argument is an lvalue or
   ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to
   ranges::cdata is expression-equivalent to:
     * /*as-const-pointer*/(ranges::data(/*possibly-const-range*/(t))).
                        (since
   The return type is equivalent to C++23)
   std::remove_reference_t<ranges::range_const_reference_t<T>>*.
   In all other cases, a call to ranges::cdata is ill-formed, which can result
   in substitution failure when the call appears in the immediate context of a
   template instantiation.
   If ranges::cdata(t) is valid, then it returns a pointer to an object
   of constant type
   (since C++23).
  Customization point objects
   The name ranges::cdata 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 __cdata_fn.
   All instances of __cdata_fn are equal. The effects of invoking different instances
   of type __cdata_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::cdata 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::cdata above, __cdata_fn models
     * std::invocable<__cdata_fn, Args...>,
     * std::invocable<const __cdata_fn, Args...>,
     * std::invocable<__cdata_fn&, Args...>, and
     * std::invocable<const __cdata_fn&, Args...>.
   Otherwise, no function call operator of __cdata_fn participates in overload
   resolution.

Example
// Run this code
 #include <cstring>
 #include <iostream>
 #include <ranges>
 #include <string>
 int main()
 {
     std::string src {"hello world!\n"};
 // std::ranges::cdata(src)[0] = 'H'; // error, src.data() is treated as read-only
     std::ranges::data(src)[0] = 'H'; // OK, src.data() is a non-const storage
     char dst[20]; // storage for a C-style string
     std::strcpy(dst, std::ranges::cdata(src));
     // [data(src), data(src) + size(src)] is guaranteed to be an NTBS
     std::cout << dst;
 }

Output:
 Hello world!

See also
   ranges::data obtains a pointer to the beginning of a contiguous range
   (C++20) (customization point object)
   data obtains the pointer to the underlying array
   (C++17) (function template)