如何解决类型依赖？（X 使用未定义的 class Y）

Question

我正在尝试为我的委托类型实现简单的快速内存管理，但遇到了我自己无法解决的循环依赖。

// Size of single bucket of delegates
static constexpr size_t _alloc_buffer_bucket_size = 128;

template <typename TFunc, typename... Args>
struct bucket;

template <typename TFunc, typename... Args>
class Delegate final : public IDelegate
{   
    . . .

    static void* operator new (size_t size)
    {
        auto bckt = _current_bucket; // gives compilation error here
    }
private:

    // Size of single bucket of delegates
    static constexpr size_t _alloc_buffer_bucket_size = 128;
    struct bucket
    {
        // stores currently occupied slots in bucket.
        size_t allocated_slots = 0;
        // when this bucket is filling with values, this 
        size_t current_slot = 0;
        std::array<Delegate<TFunc, Args...>, _alloc_buffer_bucket_size> slots = {};
    };
    // Vector of buckets.
    inline static std::vector<bucket*> _alloc_buffer = { new bucket(), };
    // This is a pointer to currently filling bucket. New delegates are added here
    // by delegate allocator.
    // When this bucket reaches _alloc_buffer_bucket_size of items, this pointer is
    // replaced with either newly allocated bucket or existing and empty bucket.
    inline static bucket* _current_bucket = &(_alloc_buffer[0]);
};

这给了我“'Delegate<void (*)(void)>::bucket::slots' uses undefined class 'std::array<Delegate<void (*)(void)>,128>'”。我怎样才能打破这种循环依赖？我尝试使用更多指针，将存储桶移出委托范围，但此错误仍然存在。

Answer 1

通过将 bucket 结构的定义移动到 Delegate 的范围之外，并将 _alloc_buffer 和 _current_bucket 变量分配到 class 之外作用域，我们就可以实现编译。我能够这样编译它：

// Size of single bucket of delegates
static constexpr std::size_t _alloc_buffer_bucket_size = 128;

template <typename TFunc, typename... Args>
struct bucket;

template <typename TFunc, typename... Args>
class Delegate final : public IDelegate
{
public:
    Delegate() = default;
    static void* operator new (std::size_t size)
    {
        auto bckt = _current_bucket; // gives compilation error here
        return nullptr; // added to get rid of the error
    }
private:
    // Vector of buckets.
    static std::vector<bucket<TFunc, Args...>*> _alloc_buffer;
    // This is a pointer to currently filling bucket. New delegates are added here
    // by delegate allocator.
    // When this bucket reaches _alloc_buffer_bucket_size of items, this pointer is
    // replaced with either newly allocated bucket or existing and empty bucket.
    static bucket<TFunc, Args...>* _current_bucket;
};

template <typename TFunc, typename... Args>
struct bucket
{
    // stores currently occupied slots in bucket.
    std::size_t allocated_slots = 0;
    // when this bucket is filling with values, this 
    std::size_t current_slot = 0;
    std::array<Delegate<TFunc, Args...>, _alloc_buffer_bucket_size> slots = {};
};

template <typename TFunc, typename... Args> 
std::vector<bucket<TFunc, Args...>*> Delegate<TFunc, Args...>::_alloc_buffer = { new bucket<TFunc, Args...>(), };

template <typename TFunc, typename... Args> 
bucket<TFunc, Args...>* Delegate<TFunc, Args...>::_current_bucket = &(_alloc_buffer[0]);

通过拆分静态变量的声明和定义，拆分出bucket类型，Delegate class在我们去实际使用的时候定义好了。在此处查看实际效果：https://godbolt.org/z/bx7hh1av9

编辑：在 MSVC 和 Clang 上测试了这个最小样本。

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