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std::ranges::data

From cppreference.com
< 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);

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(), if that 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.

Contents

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Customization point objects

The name ranges::data denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as data_ftor). All instances of data_ftor are equal. 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_ftor will satisfy std::invocable<const data_ftor&, Args...>. Otherwise, no function call operator of data_ftor participates in overload resolution.

[edit] 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.

[edit] 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;
}

Output:

Hello world!

[edit] See also

obtains a pointer to the beginning of a read-only contiguous range
(customization point object) [edit]
returns an iterator to the beginning of a range
(customization point object) [edit]
(C++17)
obtains the pointer to the underlying array
(function template) [edit]