std::ranges::data

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */ data = /* unspecified */;
}

template< class T >
    requires /* see below */
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 T is an array type and std::remove_all_extents_t<std::remove_reference_t<T>> is incomplete, then the call to ranges::data is ill-formed, no diagnostic required.

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. decay-copy(t.data()) (until C++23)auto(t.data()) (since C++23), if that expression is valid and its type is a pointer to an object type.
2. Otherwise, std::to_address(ranges::begin(std::forward<T>(t))), if the expression ranges::begin(std::forward<T>(t)) is valid and its type models std::contiguous_iterator.

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.

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

#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;
}

Hello world!

See also