std::ranges::begin

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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.

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

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

#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

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