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std::ranges:: cdata

From cppreference.net
C++
Ranges library
Range adaptors
Defined in header <ranges>
Defined in header <iterator>
inline namespace /*unspecified*/ {

inline constexpr /*unspecified*/ cdata = /*unspecified*/ ;

}
(since C++20)
(customization point object)
Call signature
template < class T >

requires /* see below */

constexpr /* see below */ cdata ( T && t ) ;
(since C++20)

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

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 >> * .

(until C++23)

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:

The return type is equivalent to 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.

(since C++23)

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. See CustomizationPointObject for details.

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

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