< cpp‎ | ranges
Ranges library
Range access
Range conversions

Range primitives

Dangling iterator handling
Range concepts

Range factories
Range adaptors
Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
(until C++23)(C++23)

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

  • const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is an lvalue reference type),
  • const T otherwise.

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:

  • /*as-const-pointer*/(ranges::data(/*possibly-const-range*/(t))).

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. 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

Otherwise, no function call operator of __cdata_fn participates in overload resolution.

[edit] Example

#include <cstring>
#include <iostream>
#include <ranges>
#include <string>
int main()
    std::string src {"hello world!\n"};
//  std::ranges::cdata(src)[0] = 'H'; // error, is treated as read-only
    std::ranges::data(src)[0] = 'H'; // OK, 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!

[edit] See also

obtains a pointer to the beginning of a contiguous range
(customization point object)[edit]
obtains the pointer to the underlying array
(function template) [edit]