Difference between revisions of "cpp/ranges/begin"

Revision as of 12:41, 4 July 2020

Returns an iterator to the first element of the argument.

Let t be an object of type T. 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 is an array type.

   If std::remove_all_extents_t<T> is incomplete, then ranges::begin(std::forward<T>(t)) is ill-formed, no diagnostic required.

2. Otherwise, std::forward<T>(t).begin(), if that expression is valid, and its return type models std::input_or_output_iterator.
3. Otherwise, begin(std::forward<T>(t)), if T is a class or enumeration type, the aforementioned unqualified call is valid, its return type models std::input_or_output_iterator, where the overload resolution is performed with the following candidates:
   • void begin(auto&) = delete;
   • void begin(const auto&) = delete;
   • any declarations of begin found by argument-dependent lookup.

In all other cases, a call to ranges::begin is ill-formed, which can result in substitution failure when ranges::begin(t) 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

• 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.

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
}

3
-5

See also