# std::ranges::data

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
inline namespace /* unspecified */ { inline constexpr /* unspecified */ data = /* unspecified */; } |  | (since C++20) (customization point object)
Call signature |  |
template< class T > requires /* see below */ constexpr std::remove_reference_t< ranges::range_reference_t<T>>* data( T&& t ); |  | (since C++20)

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

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

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