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std::ranges::begin

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< cpp‎ | ranges
 
 
Ranges library
Range access
begin
Range conversions
(C++23)

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(until C++23)(C++23)


 
Defined in header <ranges>
Defined in header <iterator>
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.
  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.

Contents

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 <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
}

[edit] 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 non-member begin found by ADL removed such machinery

[edit] See also

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