無法在模板類的成員模板函式中構造模板類的實體

我有一個這樣定義的模板類：

template<typename T> class C_SharedResource : public C_Instance
{
    // ...
private:
    std::shared_ptr<T> g_resource;
    S_AutoDestructionData g_autoDestructionData{};

    // ...
public:
    C_SharedResource() {}
    C_SharedResource(const T& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    C_SharedResource(const std::shared_ptr<T>& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    template<typename T2> C_SharedResource<T2> dynamic_cast_resource() const;

    // ...
}

dynamic_cast_resource 的定義為：

template<typename T>
template<typename T2>
C_SharedResource<T2> C_SharedResource<T>::dynamic_cast_resource() const
{
    T2* tempResourcePtr = dynamic_cast<T2*>(g_resource.get());
    if (tempResourcePtr == nullptr)
    {
        return C_SharedResource<T2>();
    }
    
    // This lines causes the compiler error.
    C_SharedResource<T2> tempSharedResource(std::dynamic_pointer_cast<T2>(g_resource), g_autoDestructionData);

    return tempSharedResource;
}

該函式背后的想法是向下轉換存盤的共享指標，然后回傳具有適當派生型別的新構造的共享資源。

使用它的一個例子：

// C_ITransformEntityComponent publicly inherits from C_IEntityComponent, which is also virtual. 
std::shared_ptr<C_ITransformEntityComponent> transformComponent = std::make_shared<C_ITransformEntityComponent>();

C_SharedResource<C_IEntityComponent> sharedTC_1(transformComponent);
C_SharedResource<C_ITransformEntityComponent> sharedTC_2;
C_SharedResource<C_ITransformEntityComponent> sharedTC_3 = sharedTC_1.dynamic_cast_resource<C_ITransformEntityComponent>();

但是這條線：

C_SharedResource<T2> tempSharedResource(std::dynamic_pointer_cast<T2>(g_resource), g_autoDestructionData);

編譯失敗，報錯：

Error C2664 'C_SharedResource<C_ITransformEntityComponent>::C_SharedResource(const T &,C_SharedResource<T>::S_AutoDestructionData)': cannot convert argument 1 from 'std::shared_ptr<C_ITransformEntityComponent>' to 'const T &'

我正在使用 Visual Studio 2022，將 C 標準設定為 C 20。

編輯：最小的可重現示例：

#include <iostream>
#include <memory>

class C_Instance
{
public:
    virtual void some_func() {}
};

template<typename T> class C_SharedResource : public C_Instance
{
public:
    struct S_AutoDestructionData
    {
        bool m_autoDestroyAllInstances = false;
        bool m_transferAutoDestructionState = false;
    };

private:
    std::shared_ptr<T> g_resource;
    S_AutoDestructionData g_autoDestructionData{};

public:
    C_SharedResource() {}
    C_SharedResource(const T& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    C_SharedResource(const std::shared_ptr<T>& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    template<typename T2> C_SharedResource<T2> dynamic_cast_resource() const;

    void some_func() override {}
};

class C_IEntityComponent
{
public:
    virtual void some_other_func() {}
};
class C_ITransformEntityComponent : public C_IEntityComponent
{
public:
    void some_other_func() override {}
};

template<typename T>
template<typename T2>
C_SharedResource<T2> C_SharedResource<T>::dynamic_cast_resource() const
{
    T2* tempResourcePtr = dynamic_cast<T2*>(g_resource.get());
    if (tempResourcePtr == nullptr)
    {
        return C_SharedResource<T2>();
    }

    C_SharedResource<T2> tempSharedResource(std::dynamic_pointer_cast<T2>(g_resource), g_autoDestructionData);

    return tempSharedResource;
}

int main()
{
    std::shared_ptr<C_ITransformEntityComponent> transformComponent = std::make_shared<C_ITransformEntityComponent>();
    C_SharedResource<C_IEntityComponent> sharedTC_1(transformComponent);
    C_SharedResource<C_ITransformEntityComponent> sharedTC_2;
    C_SharedResource<C_ITransformEntityComponent> sharedTC_3 = sharedTC_1.dynamic_cast_resource<C_ITransformEntityComponent>();
    return 0;
}

uj5u.com熱心網友回復：

您會收到編譯器錯誤，因為struct S_AutoDestructionData它是嵌套型別：

template<typename T> class C_SharedResource : public C_Instance
{
public:
    struct S_AutoDestructionData
    {
        bool m_autoDestroyAllInstances = false;
        bool m_transferAutoDestructionState = false;
    };

    // ...

    C_SharedResource(const T& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    //                                     ^ C_SharedResource<T>::S_AutoDestructionData
 
    C_SharedResource(const std::shared_ptr<T>& in_resource, S_AutoDestructionData in_autoDestructionData = { false, false });
    //                                                      ^ C_SharedResource<T>::S_AutoDestructionData

     // ...
};

所以，在這一行：

// T  = C_IEntityComponent
// T2 = C_ITransformEntityComponent
C_SharedResource<T2> tempSharedResource(std::dynamic_pointer_cast<T2>(g_resource), g_autoDestructionData);

C_SharedResource<T2>的建構式期望 aC_SharedResource<T2>::S_AutoDestructionData作為第二個引數；當你通過一個C_SharedResource<T>::S_AutoDestructionData. 那些不是同一型別。

如果您從以下位置struct S_AutoDestructionData退出class C_SharedResource：

struct S_AutoDestructionData
{
    bool m_autoDestroyAllInstances = false;
    bool m_transferAutoDestructionState = false;
};

template<typename T> class C_SharedResource : public C_Instance
{
public:

     // ...
};

struct S_AutoDestructionData將不再嵌套，您的代碼將編譯。

標籤：C 视觉工作室 班级 模板

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