NAME

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

Synopsis

Defined in header <ranges>
inline namespace /*unspecified*/ {
(since C++20)
inline constexpr /*unspecified*/ cdata = /*unspecified*/; (customization
point object)
}
Call signature
template< class T >

requires /* see below */
(since C++20)
constexpr
std::remove_reference_t<ranges::range_reference_t</*CT*/>>* cdata(
T&& t );

Returns a pointer to the first element of a contiguous range denoted by a
const-qualified argument.

Let CT be

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

a call to ranges::cdata is expression-equivalent to
ranges::data(static_cast<CT&&>(t)).

If ranges::cdata(t) is valid, then it returns a pointer to a object.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

* e and f have the same effects, and
* either both are constant subexpressions or else neither is a constant
subexpression, and
* either both are potentially-throwing or else neither is potentially-throwing
(i.e. noexcept(e) == noexcept(f)).

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)