C++ Internals

std::function Internals

Difficulty

Asked at Squarepoint Capital

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xxxxx x++ xxxxxxxxxx xxxx xxx::xxxxxxxx; x++ xxxxx xx xxxxxxx xxxx xxx xxxx xxx xxx xxxxxxx xxxx xx xxxxx, xxx xxx xxxxxxx xxx xx xxxx xxxx xxxxxx xx xx xxxxx xxx. xxx xxxxx xxxxx xxx xxxxx xxxxxx: xxxx xxxxxxx (xxx xxxxxxx xxxx xxxxxx xxx xxxxxxxx xxxx x xxxxxxxx xxxxxxxxx) xxx xxx xxxxx-xxxxxx xxxxxxxxxxxx (xxxxx xxxxxxxxx xxxx xxxxxx xxx xxxxxxx, xx xxxxxxxx x xxxxxx xxxx xxx xxxxxxxx xxxxxxx xxxxx xxxxxxx xxx xxxxxxxxx).

xxxxxxxxx xx<x(xxxx...)>:

xxxxxxx: xx xxxxxxxx xxxxxx xx xxxxxxxx = x * xxxxxx(xxxx*) xxxxx, xxxxxxxx xxxxxxx(xxx::xxx_xxxxx_x) (xxxxxxx xx xxx xxxxxxxx). x xxxxxxxx x xxxx xxxxxx(x) <= xxxxxxxx xxx xxxxxxx(x) <= xxxxxxx(xxx::xxx_xxxxx_x) xx xxxxxx xx xxx xxxxxx. xxxxxxxx xxxxxx xxxx xx xxx xxxx, xxxxxxxxx xxxx xxx xxxxxxxx xxxxxxx::xx_xxxxx xxx xxxxxxxx xxxx xxxxxxx::xx_xxxx (xxx xxxxx xxxxxx xxxxx xxxxx, xx x xxxx xxxx xxxxxxx xxxxx xxx xxxxx).
xxxxxxxxxxxx xxxx xxx xxxx-xxxxxxxxxxxxx xxxxxxxx: xxxxxxxx xxxxxxxx, xxxxxxxxx xxxxxxx, xxxxxxxx xxxxxxx (xxx xxxxxxxx xx-xxxx xxxxxxxxxxx xxxx xxxx xx xxx xxxx).
xxxx: xxxxxx xxx xxxxxx xxxxxxxx (xxxx-xxxxxx xxxxxx xxxxxxxx xxxxx xxx xxxxx; xxxxxx-xxxxxx xxxxxx xxxxxxxx xxxxxxx).
xxxx: xxxxx xxxxxxxxx. xxxx-xxxxxx xxxxx xxxxx xxx xxxxxxx; xxxxxx-xxxxxx xxxxx xxxx-xxxxxxxxx xxxx xxx xxxxxxxxxxx xxxxxx. x xxxxx-xxxx xx xx xxxxx.
xxxxxxxxxx (xxxx xxx xxxx): xxxxxxxx xxx xxxxxxx xxxxxxxx xxxxx, xxxxxxxxx xxx xxxx xxxxx xx xx xxx xxx.
xxxxxxxx xxxxxxxx xxxx(): xxxxxxx xx xxxxx. x xxxxxxx-xxxxxxxxxxx xx xx xxxxx. xxxxxxx xx xxxxx xx xx xxxxxxxxx (xx xxxx xxxx).
x xxxxxxxx()(xxxx...): xxxxxxx xxx xxxxxx xxxxxxxx. xx xx xxxxxxxxxxxx xxx-xxxxx xx xxxxxxx xxxxxxx xxxx xxxxxxx xxx xxxxx-xxxxxxxxxx xxx::xxxxxxxx xxxx.

xxxxxxxx

[]{ xx<xxxx xxxx(xxxx xxxx)> xxxxx; xxxx xxx_xxxxx = !xxxxx; xx<xxxx xxxx(xxxx xxxx)> x(+[](xxxx xxxx x) { xxxxxx x * x + x; }); xxxx xxxxxxx = xxxxxx_xxxx<xxxx>(x); xxxxxx xxx::xxxx<xxxx, xxxx xxxx>(xxx_xxxxx && xxxxxxx, x(xx)); }()
// (xxxx, xx)  (xxxxxxxx xxxxxxx xxxxxxx xxx xxxxxxx; xxxxxxx xx xx xxxxx)

[]{ xxxxxxx::xxxxx_xxxxxxxx(); xxxx xxxx xxxx = xxxx; xx<xxxx xxxx(xxxx xxxx)> x([xxxx](xxxx xxxx x) { xxxxxx xxxx + x; }); xxxxxx xxx::xxxxx<xxxx xxxx, xxxx xxxx, xxxx xxxx>(x(x), xxxxxxx::xxxx_xxxxxx(), xxxxxxx::xxxx_xxxxx()); }()
// (xxxx, x, x)  (xx x-xxxx xxxxxxx xxxx xxx xxxxx xxxxxx: xxxx xxxx xxxxxxx)

xxxxxxxxxxx

xx xxx xxxxxx xxx xxxxxxx xxxxx xx xxxxxxxxxxx; xx xxx xxxxxx xxxxxxxx xx xxx xxxxxx'x xxxxxxxxx.
xxx xxxx xxxxxxx xxx xxxxxxxxx xxxx xxxxxxx xx_xxxxx/xx_xxxx, xxxxxxx xxxx xxx xxxxxx-xx-xxxx xxxxxxxx xxx xxxx.
xxxxxx xxxxxxxxx xxxx xxxx xx xx xxxx-xxxxxxxxxxxxx xxx xxxxxxxxx xxxx xxx xxxxxxxxx'x xxxxxxxx xxxxx; xxx xxx xxxxxx xxxxx xxxxx xx xxx xxxxx.
xxxxxx xxxxx xxxxx xxxx xxxxxxx xxxxxx xxxxxxxxxxxx, xxxx, xxxx, xxxxxxxxxx xxx xxxxxxxxxxx, xxx xxxxx xxxx xxxxxx xx x xxxxxxxx xxxxxxxx xxxxxxx xxxxxxxxxxxxx.

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Language: C++Fn

#include <cstddef>
#include <new>
#include <utility>

// ...
// ...
// ...
// ...
// ...
namespace fnalloc {

inline long long& alloc_counter() { static long long c = 0; return c; }
inline long long& free_counter() { static long long c = 0; return c; }

inline void* tm_alloc(std::size_t bytes) {
    ++alloc_counter();
    return ::operator new(bytes);
}

inline void tm_free(void* p) {
    if (p == nullptr) return;
    ++free_counter();
    ::operator delete(p);
}

inline long long heap_allocs() { return alloc_counter(); }
inline long long heap_frees() { return free_counter(); }
inline void reset_counters() {
    alloc_counter() = 0;
    free_counter() = 0;
}

}  // ...

// ...
// ...
// ...
struct BigAverager {
    long long history[8];
    explicit BigAverager(long long seed) {
        for (int i = 0; i < 8; ++i) history[i] = seed + i;
    }
    long long operator()(long long x) const {
        long long s = x;
        for (long long h : history) s += h;
        return s;
    }
};

// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
// ...
template <typename Signature>
class Fn;

template <typename R, typename... Args>
class Fn<R(Args...)> {
public:
    static constexpr std::size_t kSboSize = 2 * sizeof(void*);

    Fn() noexcept {}

    template <typename F>
    Fn(F f) {
        // ...
        (void)f;
    }

    Fn(const Fn& other) {
        // ...
        (void)other;
    }

    Fn(Fn&& other) noexcept {
        // ...
        (void)other;
    }

    Fn& operator=(const Fn& other) {
        // ...
        (void)other;
        return *this;
    }

    Fn& operator=(Fn&& other) noexcept {
        // ...
        (void)other;
        return *this;
    }

    ~Fn() {
        // ...
    }

    explicit operator bool() const noexcept {
        // ...
        return false;
    }

    R operator()(Args... args) {
        // ...
        return R();
    }

private:
    alignas(std::max_align_t) unsigned char storage_[kSboSize] = {};
};

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