NAME¶

std::ranges::begin - std::ranges::begin

Synopsis¶

Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ {

inline constexpr /* unspecified */ begin = /* (since C++20)
unspecified */; (customization point object)

}
Call signature
template< class T >

requires /* see below */ (since C++20)

constexpr std::input_or_output_iterator auto begin(
T&& t );

Returns an iterator to the first element of the argument.

range-begin-end.svg

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 meaning of begin is
established as if by performing argument-dependent lookup only.

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.

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
}

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2602R2 C++20 there's machinery to prohibit certain removed such machinery
non-member begin found by ADL

See also¶

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