std::ranges:: cdata

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.

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

#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;
}

Hello world!

See also