co-cpp19/src/array19.lib/array19/WithIndex.h at develop · basicpp/co-cpp19 · GitHub

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#pragma once

#include "adlRange.h"

#include <stddef.h> // size_t

namespace array19 {

/// Iterate a container with indices

/// usage:

/// for(auto [value, index] : WithIndex{container});

template<class C> struct WithIndex {

using It = decltype(adlBegin(*static_cast<C*>(nullptr)));

using Res = decltype(**static_cast<It*>(nullptr));

// not using std::pair to avoid reference hassles

struct result {

Res value;

size_t index;

};

// minimal iterator that is just enough to run ranged for loop

struct iterator {

It it;

size_t index{};

[[nodiscard]] constexpr auto operator*() const -> result { return {*it, index}; }

[[nodiscard]] constexpr bool operator!=(const It& o) const { return it != o; }

constexpr auto operator++() -> iterator& { return (++it, ++index, *this); }

};

constexpr WithIndex(C& c) noexcept : c(&c) {}

[[nodiscard]] constexpr auto begin() -> iterator { return iterator{adlBegin(*c), {}}; }

[[nodiscard]] constexpr auto end() -> It { return adlEnd(*c); }

private:

C* c;

};

template<class C> WithIndex(C&) -> WithIndex<C>;

} // namespace array19

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