Converting user-defined containers to ranges

README.md

C++ Ranges require certain concepts to be implemented. If user-defined container doesn't refine those, it won't be treated as Range.

Here is a working minimal example using range-v3, based on this SO answer.

container.cpp

#include <iostream>
#include <iterator>
#include <range/v3/all.hpp>

struct Test {
    struct iterator;
    struct sentinel;
    int counter;
    Test() = default;
    iterator begin();
    sentinel end();

    // These are not required.
    // sentinel end() const;
    // iterator begin() const;
};

struct Test::iterator {
    using value_type = int;
    using reference = int&;
    using pointer = int*;
    using iterator_category = std::input_iterator_tag;
    using difference_type = ptrdiff_t;

    Test* test;
    iterator& operator++() {
        test->counter++;
        return *this;
    }
    void operator++(int) {
        (void)++*this;
    }
    reference operator*() { return test->counter; }
    std::add_const_t<reference> operator*() const { return test->counter; }
    bool operator!=(const iterator& rhs) const { return rhs.test != test; }
    bool operator!=(const sentinel&) const { return true; }

    template <typename T>
    bool operator==(T&& t) const { return !(*this != std::forward<T>(t)); }
};

struct Test::sentinel {
    bool operator!=(const iterator& iterator) const {
        return iterator != *this;
    }

    // This is not required:
    // bool operator!=(const sentinel&) {
    //     return true;
    // }

    template <typename T>
    bool operator==(T&& t) const { return !(*this != std::forward<T>(t)); }
};

Test::iterator Test::begin() { return {this}; }
Test::sentinel Test::end() { return {}; }

int main() {
    auto container = Test();
    static_assert(std::is_same_v<decltype(container.begin() == container.end()), bool>);
    static_assert(std::is_same_v<decltype(container.end() == container.begin()), bool>);
    static_assert(ranges::range<Test&>, "It is not a range");
    auto rng = container | ranges::views::take(10);
    for (auto n : rng) {
        std::cerr << n << std::endl;
    }
    return 0;
}

CUSTOM-CONTAINERS-AS-RANGES.md

~/tmp/range$ g++ main.cpp -std=c++17 -Irange-v3/include -o main && ./main
0
1
2
3
4
5
6
7
8
9

Tested with https://github.com/ericniebler/range-v3/tree/4b49dac2853b413440062646164cdd82866c5d16 (HEAD as of Aug 24, 2019).