NAME¶

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

Synopsis¶

Defined in header <ranges>
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, t.begin() converted to its decayed type, if that expression with
conversion is valid, and its converted type models
std::input_or_output_iterator.
3. Otherwise, begin(t) converted to its decayed type, if t has a class or
enumeration type, the aforementioned unqualified call with conversion is valid,
and its converted 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.

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::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 <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 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)