Concurrency

Double-Checked Initialization

Difficulty

Asked at XTX Markets, Squarepoint Capital

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xxxxxxxx xxxxxxxxx xxxxxxxxx xx xxxxx xxx, xxxx, xxxxxx, xxxx xxxx xxxxxxx. xx xx xxx xxxx-xxxxxxx xxxxxxxxxxx xxxxxxx xxxxx xx, xxx xxx xxxxxxx xxxxxxxxx xxxxxx xxxxx xx xxxxxx xx x xxx xxxx xxxx xxx xxxxx.

xx (xxx_ == xxxxxxx) {              // xxxx xxxxxxx xxx xxxx
    xxx::xxxx_xxxxx x(xxxxx_);
    xx (xxx_ == xxxxxxx) xxx_ = xxx xxxxxx(xxxx);
}
xxxxxx xxx_;

xxxxxxxxx xxxxxxxxxx xxxxxxxxx.

xxx(xxxx) — xxx xxxxxx, xxxxxxxx xx xx xxx xxxxx xxxx
xxxxx_xxxxx() — xxx xxxx xxxxx xxx xxxxxxxxxxx xxx

x xxxxxx xxxx xx xxxxxxxx xxxx xxxxxx xxx xxx xxxxxxxxxxxxx. xxxx xxxxxxx xxxx xxx() xx xxxx, xxx xxxxx xxx xxxx xxxx xxxx xxxx x xxxxx xxxxxxxxxxx xxxxxx. xxx xxxxxxxxxxx xxxx xxx xxxxxxx xxxx.

xxxxxxxx

xxxxxxxxxx xxxx;
xxxx.xxx(x).xxxxxxxx();   // xx
xxxx.xxxxx_xxxxx();       // x
xxxx.xxx(x).xxxxxxxx();   // xx
xxxx.xxxxx_xxxxx();       // xxxxx x

xxxxxxxxxxx

xxx xxxxxx xxxxxxx xx x xxxx xxxx xxxxxxxx xxx xxxxxxxx xxxx: xx xxxxxxxx xxxx xx x xxxxxxxx xxxxxxx xxxxxx xxxxxx xxxxx x xxxx. xxxx xxxxx xx xxxxxxxxx xxxxxxxxx.
xxx xxxxxxx xxxx xxxxxxxx xxxxx xx xxxxx xxxx "xxx xxxxxxx xxxx xxxxx xx". xxx_ = xxx xxxxxx(xxxx) xx xx xxxxxxxxxx, x xxxxxxxxxxx xxx x xxxxx, xxx xxxxxxx xxxxx xxx xxxxx xxxxxxxx xxxxxxx xxxxxx xxx xxxxxxxxxxx'x xxxxxx. x xxxxxx xxxx xxxx x xxx-xxxx xxxxxxx xx xx xxxxxxxxxx xxxxxx, xxxxx xxx xxxx xxxx, xxx xxxxx xxxxxxxxxxxxx xxxxxx. xx xxxx xxx xxxxx, xx xxxxxxx xxxxxxx.
xxxxxxx xxxxxx xxxx xxxxxxxxx xxxxx xxxx xxx xxxxxxx xxx xxxx xxxxxx. xxxxx xxxx xxxx xx xxx xxxxxxxxx; xxxx xxxx xxxxx xx.
xxx xxxxxxx xxxxxxx xxxxx xxx xxxxxx xx xxxxxxxx.

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Language: C++LazyConfig

#include <atomic>
#include <cstddef>
#include <mutex>
#include <thread>
#include <vector>

// ...
// ...
// ...
// ...
struct Config {
    static inline std::atomic<long long> builds{0};
    static void reset() { builds.store(0); }

    long long a = 0;
    long long b = 0;
    long long c = 0;

    explicit Config(long long seed) : a(seed), b(seed * 2), c(seed * 3) {
        builds.fetch_add(1, std::memory_order_relaxed);
    }

    long long checksum() const { return a + b + c; }
};

// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
class LazyConfig {
public:
    Config& get(long long seed) {
        // ...
        (void)seed;
        return *ptr_.load();
    }

    long long build_count() const { return Config::builds.load(); }

    ~LazyConfig() { delete ptr_.load(); }

private:
    std::atomic<Config*> ptr_{nullptr};
    std::mutex mutex_;
};

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