std::ranges:: view_interface

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C++
Ranges library
Views
view_interface

Range adaptors
std::ranges::view_interface
Defined in header <ranges>
template < class D >

requires std:: is_class_v < D > && std:: same_as < D, std:: remove_cv_t < D >>

class view_interface ;
(since C++20)

std::ranges::view_interface is a helper class template for defining a view interface.

view_interface is typically used with CRTP : 

class my_view : public std::ranges::view_interface<my_view>
{
public:
    auto begin() const { /*...*/ }
    auto end() const { /*...*/ }
    // empty() is provided if begin() returns a forward iterator
    // and end() returns a sentinel for it.
};

Member functions

returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range .
(public member function)
(C++23)
returns a constant iterator to the beginning of the range.
(public member function)
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function)
returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function)
gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator .
(public member function)
returns the number of elements in the derived view. Provided if it satisfies forward_range and its sentinel and iterator type satisfy sized_sentinel_for .
(public member function)
returns the first element in the derived view. Provided if it satisfies forward_range .
(public member function)
returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range .
(public member function)
returns the n th element in the derived view. Provided if it satisfies random_access_range .
(public member function)

Example

Run this code 
#include <iostream>
#include <ranges>
#include <vector>
 
template<class T, class A>
class VectorView : public std::ranges::view_interface<VectorView<T, A>>
{
public:
    VectorView() = default;
 
    VectorView(const std::vector<T, A>& vec) :
        m_begin(vec.cbegin()), m_end(vec.cend())
    {}
 
    auto begin() const { return m_begin; }
 
    auto end() const { return m_end; }
 
private:
    typename std::vector<T, A>::const_iterator m_begin{}, m_end{};
};
 
int main()
{
    std::vector<int> v = {1, 4, 9, 16};
 
    VectorView view_over_v{v};
 
    // We can iterate with begin() and end().
    for (int n : view_over_v)
        std::cout << n << ' ';
    std::cout << '\n';
 
    // We get operator[] for free when inheriting from view_interface
    // since we satisfy the random_access_range concept.
    for (std::ptrdiff_t i = 0; i != view_over_v.size(); ++i)
        std::cout << "v[" << i << "] = " << view_over_v[i] << '\n';
}

Output: 

1 4 9 16
v[0] = 1
v[1] = 4
v[2] = 9
v[3] = 16

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 3549 C++20 view_interface was required to be derived from view_base ,
which sometimes required multiple view_base subobjects in a view 		inheritance removed

See also

(C++20)
combines an iterator-sentinel pair into a view
(class template)
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