Question

以下“事件”代码段显示“纯虚函数调用”错误。但是，如标题中所述，只有在DEBUG上部署时才会发生。让我好奇的是为什么它在RELEASE上完美运行以及为什么它甚至崩溃（在DEBUG上）。或者，您可以看到代码段here。

#include <list>
#include <iostream>
#include <algorithm>

// use base class to resolve the problem of how to put into collection objects of different types
template <typename TPropertyType>
struct PropertyChangedDelegateBase
{
    virtual ~PropertyChangedDelegateBase(){};
    virtual void operator()(const TPropertyType& t) = 0;
};

template <typename THandlerOwner, typename TPropertyType>
struct PropertyChangedDelegate : public PropertyChangedDelegateBase<TPropertyType>
{
    THandlerOwner* pHandlerOwner_;

    typedef void (THandlerOwner::*TPropertyChangeHandler)(const TPropertyType&);
    TPropertyChangeHandler handler_;

public:
    PropertyChangedDelegate(THandlerOwner* pHandlerOwner, TPropertyChangeHandler handler) :
      pHandlerOwner_(pHandlerOwner), handler_(handler){}

    void operator()(const TPropertyType& t)
    {
        (pHandlerOwner_->*handler_)(t);
    }
};

template<typename TPropertyType>
class PropertyChangedEvent
{
public:
    virtual ~PropertyChangedEvent(){};

    void add(PropertyChangedDelegateBase<TPropertyType>* const d)
    {
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = std::find(observers_.begin(), observers_.end(), d);
        if(it != observers_.end())
            throw std::runtime_error("Observer already registered");

        observers_.push_back(d);
    }


    void remove(PropertyChangedDelegateBase<TPropertyType>* const d)
    {      
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = std::find(observers_.begin(), observers_.end(), d);
        if(it != observers_.end())
            observers_.remove(d);
    }  

    // notify
    void operator()(const TPropertyType& newValue)
    {
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = observers_.begin();
        for(; it != observers_.end(); ++it)
        {
            (*it)->operator()(newValue);
        }
    }

protected:
    std::list<PropertyChangedDelegateBase<TPropertyType>* const> observers_;
};

class PropertyOwner
{
    int property1_;
    float property2_;

public:
    PropertyChangedEvent<int> property1ChangedEvent;
    PropertyChangedEvent<float> property2ChangedEvent;

    PropertyOwner() :
        property1_(0),
        property2_(0.0f)
    {}  

    int property1() const {return property1_;}
    void property1(int n)
    {
        if(property1_ != n)
        {
            property1_ = n;
            property1ChangedEvent(n);
        }
    }

    float property2() const {return property2_;}
    void property2(float n)
    {
        if(property2_ != n)
        {
            property2_ = n;
            property2ChangedEvent(n);
        }
    }
};

struct PropertyObserver
{  
    void OnPropertyChanged(const int& newValue)
    {
        std::cout << "PropertyObserver::OnPropertyChanged() -> new value is: " << newValue << std::endl;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    PropertyOwner propertyOwner;
    PropertyObserver propertyObserver;

    // register observers
    PropertyChangedDelegate<PropertyObserver, int> delegate(&propertyObserver, &PropertyObserver::OnPropertyChanged);

    propertyOwner.property1ChangedEvent.add(&delegate); // Ok!
    propertyOwner.property1ChangedEvent.add(&PropertyChangedDelegate<PropertyObserver, int>(&propertyObserver, &PropertyObserver::OnPropertyChanged)); // Error: Virtual pure function call (Debug only)
    propertyOwner.property1(1);

    return getchar();
}

Answer 1

I would assume that the error is misnomer and that the problem is more likely to do with the scope that the second delegate lives. Plus declaring it outside is easier to read.

Passing around an object created on the stack rather than the heap by reference is usually a bad idea. Once the item declaration is out of scope the object is usually forgotten about.

Answer 2

The general issue is that you are binding to a temporary that gets destroyed and thus has an empty vtable and of course it generates a pure virtual call when invoked on the change of the property. If you add a dtor for the base class this is quite easy to observe:

#include <list>
#include <iostream>
#include <algorithm>

// use base class to resolve the problem of how to put into collection objects of different types
template <typename TPropertyType>
struct PropertyChangedDelegateBase
{
    virtual ~PropertyChangedDelegateBase(){};
    virtual void operator()(const TPropertyType& t) = 0;
};

template <typename THandlerOwner, typename TPropertyType>
struct PropertyChangedDelegate : public PropertyChangedDelegateBase<TPropertyType>
{
    THandlerOwner* pHandlerOwner_;

    typedef void (THandlerOwner::*TPropertyChangeHandler)(const TPropertyType&);
    TPropertyChangeHandler handler_;

public:
    PropertyChangedDelegate(THandlerOwner* pHandlerOwner, TPropertyChangeHandler handler) :
        pHandlerOwner_(pHandlerOwner), handler_(handler)
    {
        std::cout << "0x" << std::hex << this << " created!" << std::endl;
    }

    void operator()(const TPropertyType& t)
    {
        (pHandlerOwner_->*handler_)(t);
    }

    ~PropertyChangedDelegate()
    {
        std::cout << "0x" << std::hex << this << " destroyed!" << std::endl;
    }
};

template<typename TPropertyType>
class PropertyChangedEvent
{
public:
    virtual ~PropertyChangedEvent(){};

    void add(PropertyChangedDelegateBase<TPropertyType>* const d)
    {
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = std::find(observers_.begin(), observers_.end(), d);
        if (it != observers_.end())
            throw std::runtime_error("Observer already registered");

        observers_.push_back(d);
    }


    void remove(PropertyChangedDelegateBase<TPropertyType>* const d)
    {
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = std::find(observers_.begin(), observers_.end(), d);
        if (it != observers_.end())
            observers_.remove(d);
    }

    // notify
    void operator()(const TPropertyType& newValue)
    {
        std::list<PropertyChangedDelegateBase<TPropertyType>* const>::const_iterator it = observers_.begin();
        for (; it != observers_.end(); ++it)
        {
            std::cout << "Invoking 0x" << std::hex << *it << std::endl;
            (*it)->operator()(newValue);
        }
    }

protected:
    std::list<PropertyChangedDelegateBase<TPropertyType>* const> observers_;
};

class PropertyOwner
{
    int property1_;
    float property2_;

public:
    PropertyChangedEvent<int> property1ChangedEvent;
    PropertyChangedEvent<float> property2ChangedEvent;

    PropertyOwner() :
        property1_(0),
        property2_(0.0f)
    {}

    int property1() const { return property1_; }
    void property1(int n)
    {
        if (property1_ != n)
        {
            property1_ = n;
            property1ChangedEvent(n);
        }
    }

    float property2() const { return property2_; }
    void property2(float n)
    {
        if (property2_ != n)
        {
            property2_ = n;
            property2ChangedEvent(n);
        }
    }
};

struct PropertyObserver
{
    void OnPropertyChanged(const int& newValue)
    {
        std::cout << "PropertyObserver::OnPropertyChanged() -> new value is: " << newValue << std::endl;
    }
};

int main(int argc, char* argv[])
{
    PropertyOwner propertyOwner;
    PropertyObserver propertyObserver;

    // register observers
    PropertyChangedDelegate<PropertyObserver, int> delegate(&propertyObserver, &PropertyObserver::OnPropertyChanged);

    propertyOwner.property1ChangedEvent.add(&delegate); // Ok!
    propertyOwner.property1ChangedEvent.add(&PropertyChangedDelegate<PropertyObserver, int>(&propertyObserver, &PropertyObserver::OnPropertyChanged)); // Error: Virtual pure function call (Debug only)
    propertyOwner.property1(1);

    return getchar();
}

Basically you are just running into undefined behavior - the object is destroyed in both cases, but in Release the vtable is not destroyed so you get by.

Answer 3

此：

propertyOwner.property1ChangedEvent.add(
    &PropertyChangedDelegate<PropertyObserver, int>(
    &propertyObserver, 
    &PropertyObserver::OnPropertyChanged)
);

您正在捕获指向临时对象PropertyChangedDelegate<PropertyObserver, int>的指针。一旦函数调用结束并且临时被销毁，指向此对象的指针就会变为无效。取消引用此指针是未定义的行为。

在你的程序中，内存所有权关系至关重要，你应该仔细考虑它们。您需要确保所有指针都比使用它们的对象更长，可以手动：

PropertyChangedDelegate<PropertyObserver, int> delegate2 = {
    &propertyObserver, 
    &PropertyObserver::OnPropertyChanged
};

propertyOwner.property1ChangedEvent.add(&delegate2);

或使用智能指针（std::unique_ptr<>，std::shared_ptr<>）。

另一个错误：

符合C ++ 11的compier不应该允许你这样做：

std::list<PropertyChangedDelegateBase<TPropertyType>* const> observers_;

我在Visual Studio 2015中遇到的错误是：

C ++标准禁止使用const元素的容器，因为分配器格式不正确。

请参阅：Does C++11 allow vector<const T>?

<强>加成：

你的C ++风格看起来有点过时了。您可能想尝试自动类型扣除：

for(auto it = observers_.begin(); it != observers_.end(); ++it)
{
    (*it)->operator()(newValue);
}

或者，更好的是，循环范围：

for(auto observer : observers)
{
    observer(newValue);
}

您可能需要查看：

Debug上的“纯虚函数调用”错误

3 个答案: