< cpp‎ | ranges
Ranges library
Range access
Range conversions
Range primitives

Dangling iterator handling
Range concepts

Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
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.
  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.


[edit] 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.

[edit] 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.

[edit] Example

#include <iostream>
#include <ranges>
#include <vector>
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



[edit] See also

returns an iterator to the beginning of a read-only range
(customization point object) [edit]
returns an iterator to the beginning of a container or array
(function template) [edit]