Difference between revisions of "cpp/ranges/begin"

Latest revision as of 09:28, 8 September 2023

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 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.

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.

[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.

[edit] See also