c++ - 为什么定义静态成员变量时不遵循定义顺序？

Question

我知道不同翻译单元的静态变量初始化顺序的问题。但是，我的问题是在一个翻译单元内，实际上是在一个结构内:

template <int size>
struct SlidingTile {
    using AllActions = std::array<int, size>;
    using AllMDDeltas = std::array<int, size>;

    int mdDelta(int i) const {
        return mdDeltas_[i];
    }

    static AllActions computeAllActions() {
        std::cout << "computeAllActions" << std::endl;
        AllActions res;
        for (int i = 0; i < size; ++i) res[i] = i;
        return res;
    }

    static AllMDDeltas computeAllMDDeltas() {
        std::cout << "Entered computeAllMDDeltas" << std::endl;
        AllActions res;
        for (int i = 0; i < size; ++i) res[i] = 10 * allActions_[i];
        std::cout << "Exiting computeAllMDDeltas" << std::endl;
        return res;
    }

private:
    static const AllActions allActions_;
    static const AllMDDeltas mdDeltas_;
};

template <int size>
const typename SlidingTile<size>::AllActions
    SlidingTile<size>::allActions_ = SlidingTile<size>::computeAllActions();

template <int size>
const typename SlidingTile<size>::AllMDDeltas
    SlidingTile<size>::mdDeltas_ = SlidingTile<size>::computeAllMDDeltas();

int main() {
    SlidingTile<3> s;
    std::cout << s.mdDelta(2) << std::endl;
    return 0;
}

输出是:

Entered computeAllMDDeltas
Exiting computeAllMDDeltas
computeAllActions

令我惊讶的是，computeAllMDDeltas 在 computeAllActions 之前被调用，因此 allActions_ 在 computeAllMDDeltas 。有趣的是，即使在 computeAllMDDeltas 中使用了 allActions_，也不会调用 computeAllActions。

为什么会发生这种情况，在这种情况下建议采取什么方式？

Answer 1

How come the definition order is not followed when defining static member variables?

因为标准说初始化是无序的:

[basic.start.init] /2 (N4140 standard draft)

... Definitions of explicitly specialized class template static data members have ordered initialization. Other class template static data members (i.e., implicitly or explicitly instantiated specializations) have unordered initialization. ...

---

what is the advised way in this situation?

与跨翻译单元的初始化相同:Construct On First Use 成语:

struct SlidingTile {
    // ...
private:
    static const AllActions& allActions() {
        static const AllActions instance = computeAllActions();
        return instance;
    }
    static const AllMDDeltas& mdDeltas() {
        static const AllMDDeltas instance = computeAllMDDeltas();
        return instance;
    }
};