C 库的 C++/CLI 类包装器 - 回调

Question

我正在使用 C++/CLI 包装 C 库。 C 库设计为从非托管 C++ 类中使用。这意味着库函数接受 C++ 对象指针，然后在回调中提供该指针。这使得回调代码能够将请求重定向到调用 C++ 对象中的适当事件函数。

实际函数非常复杂，因此我将问题空间简化为几个基本项：

// C library function signature
void CLibFunc(CLIBCALLBACK *callback, void *caller);

// C callback signature
// Second parameter is meant to point to the calling C++ object
typedef int (__stdcall CLIBCALLBACK) (int param1, void *caller);

// C callback implementation
int CallBackImpl(int param1, void* caller)
{
    // Need to call the ManagedCaller's EventFunction from here
    // ???
}

// C++/CLI caller class
public ref class ManagedCaller
{
    public:
        void CallerFunction(void)
        {
            // Call the C library function
            // Need to pass some kind of this class pointer that refers to this object
            CLibFunc(CallBackImpl, ????);
        }

        void EventFunction(param1)
        {
        }
}

现在需要从托管 C++ 类调用 C 库函数。在 C++/CLI 下，垃圾收集器在内存中移动对象，因此向类传递简单的固定指针不再起作用。我可以通过固定对象来解决问题，但不建议这样做，因为它会导致内存碎片。似乎另一个选择是使用 auto_gcroot 指针，但我对托管 C++ 相当陌生，我不知道如何完成这项工作。

有谁知道如何进行这项工作？应该将什么样的指针传递给 C 函数？回调实现应该如何重定向到调用对象的事件函数？

Answer 1

这恰好与我现在正在做的事情相似。

以下是关于使用 C++ 类提供本机回调的博客文章： http://blogs.microsoft.co.il/blogs/alon/archive/2007/05/29/Native-Callback.aspx

我不熟悉从 C 调用 C++ 成员函数，但我已经为另一个 C++ 类做了一个接口（抽象基）类以进行回调（类似于本文）。下面是我提供桥的一个基本示例：

// Interface (abstract base) class providing the callback
class IProvider {
public:
    virtual ~IProvider() {}
    virtual void Callback() = 0;
};

// User class of the callback
class CUser {   
    IProvider * m_pProvider;
public:
    CUser(IProvider * pProvider) {
        m_pProvider = pProvider;
    }
    void DoSomething() {
        m_pProvider->Callback();
    }
};

// Implementation of the interface class
class CHelloWorldProvider : public IProvider {
    void Callback() {
        printf("Hello World!");
    }
};

// Usage of the callback provider in a pure native setting
void PureNativeUsage() {
    CHelloWorldProvider oProvider;
    CUser oUser(&oProvider);
    oUser.DoSomething();
}

现在，为了使其可用于提供程序的托管实现，我们必须创建一系列提供桥的类。

// Where gcroot is defined
#include <vcclr.h>

// Managed provider interface class 
public interface class IManagedProvider {
    void Callback();
};

// Native bridge class that can be passed to the user
class CProviderBridge : public IProvider {
    // Give the managed class full access
    friend ref class ManagedProviderBase;

    // Store a reference to the managed object for callback redirects
    gcroot<IManagedProvider ^> m_rManaged;

public:
    void Callback(){
        m_rManaged->Callback();
    }
};

// Managed provider base class, this provides a managed base class for extending
public ref class ManagedProviderBase abstract : public IManagedProvider {
    // Pointer to the native bridge object
    CProviderBridge * m_pNative;

protected:
    ManagedProviderBase() {
        // Create the native bridge object and set the managed reference
        m_pNative = new CProviderBridge();
        m_pNative->m_rManaged = this;
    }

public:
    ~ManagedProviderBase() {
        delete m_pNative;
    }

    // Returns a pointer to the native provider object
    IProvider * GetProvider() {
        return m_pNative;
    }

    // Makes the deriving class implement the function
    virtual void Callback() = 0;
};

// Pure managed provider implementation (this could also be declared in another library and/or in C#/VB.net)
public ref class ManagedHelloWorldProvider : public ManagedProviderBase {
public:
    virtual void Callback() override {
        Console::Write("Hello World");
    }
};

// Usage of the managed provider from the native user
void MixedUsage() {
    ManagedHelloWorldProvider ^ rManagedProvider = gcnew ManagedHelloWorldProvider;
    CUser oUser(rManagedProvider->GetProvider());
    oUser.DoSomething();
}

---

编辑：添加了代码来显示我使用的托管接口类示例。

这是我的示例的修改版本，可以根据上面的 CLibFunc 使用。这是假设 C 函数执行回调的方式是准确的。

此外，这可能会稍微精简一些，具体取决于回调类的参与程度以及您需要的扩展自由度。

// Where gcroot is defined
#include <vcclr.h>

// C callback signature
// Second parameter is meant to point to the calling C++ object
typedef int (__stdcall CLIBCALLBACK) (int param1, void *caller);

// C library function
void CLibFunc(CLIBCALLBACK *callback, void *caller) {
    // Do some work
    (*callback)(1234, caller);
    // Do more work
}

// Managed caller interface class 
public interface class IManagedCaller {
    void EventFunction(int param1);
};

// C++ native bridge struct
struct CCallerBridge {
    // Give the managed class full access
    friend ref class ManagedCaller;

    // Store a reference to the managed object for callback redirects
    gcroot<IManagedCaller ^> m_rManaged;

public:
    // Cast the caller to the native bridge and call managed event function
    // Note: This must be __stdcall to prevent function call stack corruption
    static int __stdcall CallBackImpl(int param1, void * caller) {
        CCallerBridge * pCaller = (CCallerBridge *) caller;
        pCaller->m_rManaged->EventFunction(param1);
        return 0;
    }
};

// C++/CLI caller class
public ref class ManagedCaller : public IManagedCaller {
    // Pointer to the native bridge object
    CCallerBridge * m_pNative;

public:
    ManagedCaller() {
        // Create the native bridge object and set the managed reference
        m_pNative = new CCallerBridge();
        m_pNative->m_rManaged = this;
    }
    ~ManagedCaller() {
        delete m_pNative;
    }

    // Calls the C library function
    void CallerFunction() {
        CLibFunc(CCallerBridge::CallBackImpl, m_pNative);
    }

    // Managed callback function
    virtual void EventFunction(int param1) {
        Console::WriteLine(param1);
    }
};

// Usage
int main(array<System::String ^> ^args) {
    ManagedCaller ^ oCaller = gcnew ManagedCaller();
    oCaller->CallerFunction();
    return 0;
}