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std::ranges::cdata(3) C++ Standard Libary std::ranges::cdata(3)


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


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
(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
A call to ranges::cdata is expression-equivalent to

The return type is equivalent to

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 C++23)

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


// 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, 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!

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)