std::ranges::begin

 

C++

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Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	views::counted
Adaptors
views::all_tviews::all join_viewviews::join take_viewviews::take basic_istream_viewistream_view	ref_view                                split_viewviews::split take_while_viewviews::take_while elements_viewviews::elements	common_viewviews::common transform_viewviews::transform drop_viewviews::drop keys_viewviews::keys	reverse_viewviews::reverse filter_viewviews::filter drop_while_viewviews::drop_while values_viewviews::values

 

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

Returns an iterator to the first element of the argument.

Let t be an object of type T. If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to ranges::begin is expression-equivalent to:

1. t + 0 if T is an array type.

   If std::remove_all_extents_t<T> is incomplete, then ranges::begin(std::forward<T>(t)) is ill-formed, no diagnostic required.

2. Otherwise, std::forward<T>(t).begin(), if that expression is valid, and its return type models std::input_or_output_iterator.
3. Otherwise, begin(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, its return type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
   • void begin(auto&) = delete;
   • void begin(const auto&) = delete;
   • any declarations of begin found by argument-dependent lookup.

In all other cases, a call to ranges::begin is ill-formed, which can result in substitution failure when ranges::begin(t) 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, 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.

Customization point objects

The name ranges::begin denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as begin_ftor). All instances of begin_ftor are equal. Thus, ranges::begin 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::begin above, begin_ftor will satisfy std::invocable<const begin_ftor&, Args...>. Otherwise, no function call operator of begin_ftor 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::begin is ill-formed, which also results in substitution failure.

The return type models std::input_or_output_iterator in all cases.

Example

Run this code

#include <iostream>
#include <vector>
#include <ranges>
 
int main() 
{
    std::vector<int> v = { 3, 1, 4 };
    auto vi = std::ranges::begin(v);
    std::cout << *vi << '\n';
    *vi = 42; // OK
 
    int a[] = { -5, 10, 15 };
    auto ai = std::ranges::begin(a);
    std::cout << *ai << '\n';
    *ai = 42; // OK
}

Output:

3
-5

See also

ranges::cbegin (C++20)	returns an iterator to the beginning of a read-only range (customization point object)
begincbegin (C++11)(C++14)	returns an iterator to the beginning of a container or array (function template)