Visit a Tagged Union

Difficulty

Asked at Squarepoint Capital, XTX Markets

Theory: Data Structures You Implement, Not Import

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Language: C++23Message

#include <cstddef>
#include <new>
#include <string>
#include <type_traits>
#include <utility>

// ...
// ...
// ...
// ...
struct Lifetime {
    static inline long long constructed = 0;
    static inline long long destroyed = 0;
    static void reset() { constructed = 0; destroyed = 0; }
    static long long live() { return constructed - destroyed; }
};

struct Quote : Lifetime {
    long long bid = 0;
    long long ask = 0;
    Quote(long long b, long long a) : bid(b), ask(a) { ++constructed; }
    Quote(const Quote& other) : bid(other.bid), ask(other.ask) { ++constructed; }
    ~Quote() { ++destroyed; }
};

struct Trade : Lifetime {
    long long price = 0;
    long long qty = 0;
    Trade(long long p, long long q) : price(p), qty(q) { ++constructed; }
    Trade(const Trade& other) : price(other.price), qty(other.qty) { ++constructed; }
    ~Trade() { ++destroyed; }
};

struct Cancel : Lifetime {
    long long order_id = 0;
    explicit Cancel(long long id) : order_id(id) { ++constructed; }
    Cancel(const Cancel& other) : order_id(other.order_id) { ++constructed; }
    ~Cancel() { ++destroyed; }
};

// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
class Message {
public:
    explicit Message(const Quote& value) {
        // ...
        (void)value;
    }

    explicit Message(const Trade& value) {
        // ...
        (void)value;
    }

    explicit Message(const Cancel& value) {
        // ...
        (void)value;
    }

    ~Message() {
        // ...
    }

    Message(const Message&) = delete;
    Message& operator=(const Message&) = delete;

    long long index() const { return tag_; }

    template <typename F>
    auto visit(F&& f) const {
        // ...
        (void)f;
        return decltype(f(std::declval<const Cancel&>())){};
    }

private:
    static constexpr std::size_t kSize =
        sizeof(Quote) > sizeof(Trade)
            ? (sizeof(Quote) > sizeof(Cancel) ? sizeof(Quote) : sizeof(Cancel))
            : (sizeof(Trade) > sizeof(Cancel) ? sizeof(Trade) : sizeof(Cancel));

    alignas(Quote) alignas(Trade) alignas(Cancel) std::byte storage_[kSize];
    long long tag_ = -1;
};

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