# std::ranges::begin

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< cpp‎ | ranges

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)
begin cbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)

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