c++引用与虚函数，模拟实现自己的虚函数表-优快云博客

本文探讨了C++中虚函数表的概念及其工作原理。通过具体的代码示例，展示了基类与派生类间的虚函数调用机制，并比较了引用与对象赋值的不同效果。此外，还提供了一个使用C语言实现类似虚函数表机制的例子。

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#include <iostream>
using namespace std;

class Base
{
public:

    Base()
    {
        cout<<"Generate a Base"<<endl;
    }
    virtual ~Base()
    {	
        cout<<"Destroy a Base"<<endl;
    }


    virtual void reclaim()
    {
        cout<<"reclaim a Base"<<endl;
    }
};

class Derived:public Base
{
public:

    Derived():Base()
    {
        tmp = new int[10];
        cout<<"Generate a Derived"<<endl;
    }
    virtual ~Derived()
    {	
        delete []tmp;
        cout<<"Destroy a Derived"<<endl;
    }
    virtual void reclaim()
    {
        cout<<"reclaim a Derived"<<endl;
    }
private:
    int* tmp;
};




int main()
{
    {
        Derived d;
        Base& b = d;
        b.reclaim();
    }
    
    char c_exit;
    cin>>c_exit;
    
}

53行Base& b=d;

b与d代表同一个对象，拥有同一个虚函数表vtable。

如果改成Base b=d;

b与d不是同一个对象，而且拥有不同类型，拥有不同的虚函数表vtable。

模拟虚函数表，引用自mattew wilson <imperfect c++>;实际上vlc中，用C语言，通过类似的方法模拟了虚函数表。

//IObject.h

#include <assert.h>

# ifdef __cplusplus
extern "C"
{
#endif


struct IObjectVTable
{
    void (*SetName)(struct IObject *obj, char const *s);
    char const *(*GetName)(struct IObject const *obj);
};

struct IObject
{
    struct IObjectVTable *const vtable;

# ifdef __cplusplus
protected:
    typedef struct IObjectVTable    vtable_t;

protected:
    IObject(vtable_t *vt)
        : vtable(vt)
    {}
    IObject(IObject const &, vtable_t *vt)
        : vtable(vt)
    {}

public:
    inline void SetName(char const *s)
    {
        assert(NULL != vtable);

        vtable->SetName(this, s);
    }
    inline char const *GetName() const
    {
        assert(NULL != vtable);

        return vtable->GetName(this);
    }
private:
    IObject(IObject const &rhs);
    IObject &operator =(IObject const &rhs);
protected:
    ~IObject()
    {

    }
# endif /* __cplusplus */
};

IObject *make_Object(char const *s);
void release_Object(IObject*& obj);

# ifdef __cplusplus
};
#endif

//ObjectServer.cpp

#include <stdio.h>

#include "IObject.h"

class Object1
    : public IObject
{
public:
    typedef Object1     class_type;

public:
    virtual void SetName(char const *s)
    {
        m_name = s;
    }
    virtual char const *GetName() const
    {
        return m_name;
    }

public:
    Object1()
        : IObject(GetVTable())
    {}
    Object1(Object1 const &rhs)
        : IObject(GetVTable())
        , m_name(rhs.m_name)
    {}
public:
    ~Object1()
    {

    }

private:
    static void SetName_(IObject *this_, char const *s)
    {
        static_cast<class_type*>(this_)->SetName(s);
    }
    static char const *GetName_(IObject const *this_)
    {
        return static_cast<class_type const*>(this_)->GetName();
    }

    static vtable_t *GetVTable()
    {
        static vtable_t s_vt = MakeVTable();

        return &s_vt;
    }

    static vtable_t MakeVTable()
    {
        vtable_t vt = { SetName_, GetName_ };

        return vt;
    }


private:
    char const *m_name;
};

IObject *make_Object(char const *s)
{
    IObject *obj = new Object1();
    obj->SetName(s);
    return obj;
}
void release_Object(IObject*& obj)
{
    Object1* p_b = static_cast<Object1*>(obj);
    delete p_b;
    obj = NULL;
}

int main(int argc, char** argv)
{
    char const *strings[] = 
    {
        "Reginald Perrin"
        ,   "Archie Goodwin"
        ,   "Scott Tracy"
    };

    for(int i = 0; i < 3; ++i)
    {
        printf("Calling make_Object()\n");
        IObject *obj = make_Object(strings[i]);

        printf("%s\n", obj->GetName());
        release_Object(obj);
    }

    return 0;
}