# std::ranges::transform_view<V,F>::iterator

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< cpp‎ | ranges‎ | transform view

template< bool Const > class /*iterator*/ |  | (since C++20) (exposition only*)

The return type of `transform_view::begin`, and of `transform_view::end` when the underlying view is a `common_range`.

The type /*iterator*/<true> is returned by the const-qualified overloads. The type /*iterator*/<false> is returned by the non-const-qualified overloads.

The name of this class template (shown here as `iterator`) is unspecified.

Typical implementation holds two data members: an iterator into (possibly const-qualified) `V`, and a pointer to parent `transform_view`.

### Member types

Member type | Definition
`Parent` (private) | const ranges::transform_view<V, F> if Const is `true`, otherwise ranges::transform_view<V, F>. The name is for exposition only.
`Base` (private) | const V if Const is `true`, otherwise V. The name is for exposition only.
`iterator_concept` | std::random_access_iterator_tag if `Base` models `random_access_range`, std::bidirectional_iterator_tag if `Base` models `bidirectional_range`, std::forward_iterator_tag if `Base` models `forward_range`, std::input_iterator_tag otherwise.
`iterator_category` | Not defined if `Base` does not model `forward_range`. Otherwise, if std::invoke_result_t<F&, ranges::range_reference_t<Base>> is not a reference, std::input_iterator_tag. Else, let `C` be std::iterator_traits<ranges::iterator_t<Base>>::iterator_category. If `C` is std::contiguous_iterator_tag, the type is std::random_access_iterator_tag; otherwise, the type is `C`.
`value_type` | std::remove_cvref_t<std::invoke_result_t<F&, ranges::range_reference_t<Base>>>
`difference_type` | ranges::range_difference_t<Base>

### Member functions

(constructor) (C++20) | constructs an iterator (public member function)
base (C++20) | returns the underlying iterator (public member function)
operator* (C++20) | accesses the transformed element (public member function)
operator[] (C++20) | accesses an element by index (public member function)
operator++operator++(int)operator--operator--(int)operator+=operator-= (C++20) | advances or decrements the underlying iterator (public member function)

### Non-member functions

operator==operator<operator>operator<=operator>=operator<=> (C++20) | compares the underlying iterators (function)
operator+operator- (C++20) | performs iterator arithmetic (function)
iter_move (C++20) | obtains an rvalue reference to the transformed element (function)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2259R1 | C++20 | member `iterator_category` is always defined | defined only if `Base` models `forward_range`
LWG 3555 | C++20 | the definition of `iterator_concept` ignores const | made to consider
LWG 3798 | C++20 | `iterator_category` is input-only if transformation result is rvalue reference | may have a stronger category

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