std::ranges::view_interface - cppreference.com (original) (raw)
std::ranges::view_interface
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 { public: auto begin() const { /.../ } auto end() const { /.../ } // empty() is provided if begin() returns a forward iterator // and end() returns a sentinel for it. };
[edit] Member functions
| empty | returns whether the derived view is empty, provided only if it satisfies sized_range or forward_range (public member function) [edit] |
|---|---|
| cbegin(C++23) | returns a constant iterator to the beginning of the range (public member function) [edit] |
| cend(C++23) | returns a sentinel for the constant iterator of the range (public member function) [edit] |
| operator bool | returns whether the derived view is not empty, provided only if ranges::empty is applicable to it (public member function) [edit] |
| data | gets the address of derived view's data, provided only if its iterator type satisfies contiguous_iterator (public member function) [edit] |
| size | 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] |
| front | returns the first element in the derived view, provided if it satisfies forward_range (public member function) [edit] |
| back | returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range (public member function) [edit] |
| operator[] | returns the nth element in the derived view, provided only if it satisfies random_access_range (public member function) [edit] |
[edit] Example
#include #include #include 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 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
[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 |