std::ranges::data

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

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 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 (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)