Optional Without Heap

Difficulty

Asked at Headlands Technologies, Squarepoint Capital

Theory: Data Structures You Implement, Not Import

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Language: C++23Optional

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

// ...
// ...
// ...
// ...
namespace probe {
inline long long& counter() {
    static long long calls = 0;
    return calls;
}
inline long long allocations() { return counter(); }
inline void reset() { counter() = 0; }
}  // ...

void* operator new(std::size_t bytes) {
    ++probe::counter();
    void* p = std::malloc(bytes ? bytes : 1);
    if (!p) throw std::bad_alloc();
    return p;
}
void* operator new[](std::size_t bytes) { return ::operator new(bytes); }
void operator delete(void* p) noexcept { std::free(p); }
void operator delete[](void* p) noexcept { std::free(p); }
void operator delete(void* p, std::size_t) noexcept { std::free(p); }
void operator delete[](void* p, std::size_t) noexcept { std::free(p); }

// ...
// ...
// ...
// ...
// ...
struct Tracked {
    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; }

    long long value = 0;

    explicit Tracked(long long v) : value(v) { ++constructed; }
    Tracked(const Tracked& other) : value(other.value) { ++constructed; }
    ~Tracked() { ++destroyed; }
    Tracked& operator=(const Tracked&) = delete;
};

// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
template <typename T>
class Optional {
public:
    Optional() = default;

    ~Optional() {
        // ...
    }

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

    template <typename... Args>
    T& emplace(Args&&... args) {
        // ...
        return *reinterpret_cast<T*>(&storage_);
    }

    void reset() {
        // ...
    }

    bool has_value() const { return engaged_; }
    explicit operator bool() const { return engaged_; }

    T& value() {
        // ...
        return *reinterpret_cast<T*>(&storage_);
    }

    const T& value() const {
        // ...
        return *reinterpret_cast<const T*>(&storage_);
    }

    T value_or(T fallback) const {
        // ...
        return fallback;
    }

private:
    alignas(T) std::byte storage_[sizeof(T)];
    bool engaged_ = false;
};

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