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std::ranges::view_interface

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< cpp‎ | ranges
 
 
 
std::ranges::view_interface
Member functions
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
 
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.
};

Contents

[edit] Member functions

(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_range.
(public member function) [edit]
Returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function) [edit]
(C++20)
Gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator.
(public member function) [edit]
(C++20)
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) [edit]
(C++20)
Returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function) [edit]
(C++20)
Returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function) [edit]
Returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function) [edit]

[edit] Example

#include <ranges>
#include <vector>
#include <iostream>
 
template <class T, class A>
class VectorView : public std::ranges::view_interface<VectorView<T, A>> {
public:
    // We need to define a default constructor to satisfy the view concept.
    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 inherting 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

[edit] 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

[edit] See also

combines an iterator-sentinel pair into a view
(class template) [edit]