NAME

std::ranges::data - std::ranges::data

Synopsis

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

requires /* see below */ (since C++20)

constexpr std::remove_reference_t<ranges::range_reference_t<T>>*
data( T&& t );

Returns a pointer to the first element of a contiguous range.

If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>>
is true, a call to ranges::data is expression-equivalent to:

1. std::forward<T>(t).data() converted to its decayed type, if that converted
expression is valid, and its return type is a pointer to an object type.
2. Otherwise, std::to_address(ranges::begin(std::forward<T>(t))), if
ranges::begin(std::forward<T>(t)) is valid and returns a type that models
std::contiguous_iterator.

If std::remove_all_extents_t<std::remove_reference_t<T>> is
incomplete, then ranges::data(std::forward<T>(t)) is ill-formed, no
diagnostic required.

In all other cases, a call to ranges::data is ill-formed, which can result in
substitution failure when ranges::data(e) appears in the immediate context of a
template instantiation.

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::data 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 __data_fn.

All instances of __data_fn are equal. The effects of invoking different instances of
type __data_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::data 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::data above, __data_fn models

* std::invocable<__data_fn, Args...>,
* std::invocable<const __data_fn, Args...>,
* std::invocable<__data_fn&, Args...>, and
* std::invocable<const __data_fn&, Args...>.

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

Notes

If the argument is an rvalue (i.e. T is an object type) and
ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to
ranges::data is ill-formed, which also results in substitution failure.

If ranges::data(e) is valid for an expression e, then it returns a pointer to an
object.

The C++20 standard requires that if the underlying data function call returns a
prvalue, the return value is move-constructed from the materialized temporary
object. All implementations directly return the prvalue instead. The requirement is
corrected by the post-C++20 proposal P0849R8 to match the implementations.

Example

// Run this code

#include <cstring>
#include <iostream>
#include <ranges>
#include <string>

int main()
{
std::string s {"Hello world!\n"};

char a[20]; // storage for a C-style string
std::strcpy(a, std::ranges::data(s));
// [data(s), data(s) + size(s)] is guaranteed to be an NTBS

std::cout << a;
}

Output:

Hello world!

See also

ranges::cdata obtains a pointer to the beginning of a read-only contiguous range
(C++20) (customization point object)
ranges::begin returns an iterator to the beginning of a range
(C++20) (customization point object)
data obtains the pointer to the underlying array
(C++17) (function template)