std::move Traps

Difficulty

Asked at Squarepoint Capital, Akuna Capital

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Language: C++23Tracer

#include <utility>

// ...
// ...
// ...
// ...
// ...
// ...
namespace movecheck {

inline long long& copy_counter() { static long long c = 0; return c; }
inline long long& move_counter() { static long long c = 0; return c; }

inline long long copies() { return copy_counter(); }
inline long long moves() { return move_counter(); }
inline void reset_counters() {
    copy_counter() = 0;
    move_counter() = 0;
}

}  // ...

struct Tracer {
    long long value;

    Tracer() : value(0) {}
    explicit Tracer(long long v) : value(v) {}

    Tracer(const Tracer& other) : value(other.value) { ++movecheck::copy_counter(); }
    Tracer(Tracer&& other) noexcept : value(other.value) {
        other.value = -1;
        ++movecheck::move_counter();
    }
    Tracer& operator=(const Tracer& other) {
        value = other.value;
        ++movecheck::copy_counter();
        return *this;
    }
    Tracer& operator=(Tracer&& other) noexcept {
        value = other.value;
        other.value = -1;
        ++movecheck::move_counter();
        return *this;
    }
};

// ...
// ...
struct Box {
    Tracer inner;
    void put(const Tracer& t) { inner = t; }
    void put(Tracer&& t) { inner = std::move(t); }
};

// ...
// ...
// ...

// ...
// ...
struct Holder {
    Tracer stored;

    void set(Tracer t) {
        // ...
        (void)t;
    }
};

// ...
// ...
// ...
// ...
inline Tracer make_local(long long n) {
    // ...
    (void)n;
    return Tracer(0);
}

// ...
// ...
template <typename T>
void relay(Box& box, T&& t) {
    // ...
    (void)box;
    (void)t;
}

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