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Showing content from https://en.cppreference.com/w/cpp/algorithm/../string/../ranges/view_interface.html below:

std::ranges::view_interface - cppreference.com

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.
};

#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

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

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