std::ranges::cdata

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

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*/ cdata = /*unspecified*/; }		(since C++20) (customization point object)
Call signature
template< class T >     requires /* see below */ constexpr std::remove_reference_t<ranges::range_reference_t</*CT*/>>* cdata( T&& t );		(since C++20)

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 (C++20)	obtains a pointer to the beginning of a contiguous range (customization point object)
data (C++17)	obtains the pointer to the underlying array (function template)