std::ranges::begin

 

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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_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (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
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
Range generators
std::generator (C++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

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 );		(since C++20)

Returns an iterator to the first element of the argument.

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 has an array type.
   • If std::remove_all_extents_t<std::remove_reference_t<T>> is incomplete, then the call to ranges::begin is ill-formed, no diagnostic required.
2. Otherwise, decay-copy(t.begin()) (until C++23)auto(t.begin()) (since C++23), if that expression is valid and its type models std::input_or_output_iterator.
3. Otherwise, decay-copy(begin(t)) (until C++23)auto(begin(t)) (since C++23), if T is a class or enumeration type, that expression is valid and its type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
   • any declarations of begin found by argument-dependent lookup.
   • void begin(auto&) = delete;
   • void begin(const auto&) = delete;

In all other cases, a call to ranges::begin is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation.

Customization point objects

The name ranges::begin 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 __begin_fn.

All instances of __begin_fn are equal. The effects of invoking different instances of type __begin_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::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_fn models

• std::invocable<__begin_fn, Args...>,
• std::invocable<const __begin_fn, Args...>,
• std::invocable<__begin_fn&, Args...>, and
• std::invocable<const __begin_fn&, Args...>.
  

Otherwise, no function call operator of __begin_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::begin is ill-formed, which also results in substitution failure.

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

The C++20 standard requires that if the underlying begin 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 <cassert>
#include <ranges>
#include <vector>
 
int main() 
{
    std::vector v{3, 1, 4};
    auto vi = std::ranges::begin(v);
    auto vci = std::ranges::cbegin(v);
    assert(*vi == 3 and *vi == *vci);
    ++vi;
    ++vci; // OK: vci is modifiable object
    *vi = 42; // OK: vi points to mutable element
    // *vci = 13; // Error: vci points to immutable element
 
    int a[]{-5, 10, 15};
    auto ai = std::ranges::begin(a); // works with C-arrays as well
    assert(*ai == -5);
    *ai = 42; // OK
}

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)