std::ranges::begin

< cpp‎ | ranges
 Defined in header  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 is true, then a call to ranges::begin is expression-equivalent to:

1. t + 0 if t has an array type.
If is incomplete, then the call to ranges::begin is ill-formed, no diagnostic required.
2. Otherwise, t.begin() converted to its decayed type, if that expression with conversion is valid, and its converted type models std::input_or_output_iterator.
3. Otherwise, begin(t) converted to its decayed type, if t has a class or enumeration type, the aforementioned unqualified call with conversion is valid, and its converted 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.

Contents

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

 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 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

#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
}

Output:

3
-5