std::ranges::begin - cppreference.com (original) (raw)

| Defined in header | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------------------------------------ | | 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 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. See CustomizationPointObject for details.

[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 #include #include   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

| | 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] |