c++ - 分配给定指针的另一个对象

Question

我想我需要分配给定指针类型的另一个对象，它派生自(或是)一个基类。

例如，我有这个类 CChunk，及其派生类 CMODChunk。

我希望，在我向其传递指向 CChunk 的指针(可能是 CChunk、CMODChunk 或任何其他派生的)的函数中，我可以分配该给定类型的另一个对象。就是这样。基本上，我想知道指针指向的类型。

我写了一个很长的测试代码来尝试理解 typeof、typeid 以及更多关于虚拟的东西......

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <typeinfo>
using namespace std;

class base_virtual
    {
    public:
    base_virtual ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    virtual void dummy( void )
        {
        }
    base_virtual *Alloc ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        return new typeof (*this);
        };
    void GetTypeInfo()
        {
        printf("%sname %s\n",__PRETTY_FUNCTION__, typeid( *this).name());
        }
    };

class der_virtual_1: public base_virtual
    {
    public:
    der_virtual_1 ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    base_virtual *Alloc ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        return new typeof (*this);
        };
    };

class der_virtual_2 : public base_virtual
    {
    public:
    der_virtual_2 ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    };

class base
    {
    public:
    base ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    void dummy( void )
        {
        }
    base *Alloc ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        return new typeof (*this);
        };
    void GetTypeInfo()
        {
        printf("%s %s\n",__PRETTY_FUNCTION__, typeid( *this).name());
        }
    };

class der_1 : public base
    {
    public:
    der_1 ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    base *Alloc ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        return new typeof (*this);
        };
    };

class der_2 : public base
    {
    public:
    der_2 ( void )
        {
        printf("%s\n",__PRETTY_FUNCTION__);
        }
    };

int main ( int argc, char *argv[] )
    {
    printf("\n\n%s Criacao de objectos\n",__PRETTY_FUNCTION__);
    printf("\n");
    printf("%s base virtual\n",__PRETTY_FUNCTION__);
    base_virtual bv1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bv1).name());
    bv1.GetTypeInfo();

    printf("%s base nao virtual\n",__PRETTY_FUNCTION__);
    base b1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(b1).name());
    b1.GetTypeInfo();

    printf("\n");
    printf("%s derivado 1 virtual\n",__PRETTY_FUNCTION__);
    der_virtual_1 dv1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(dv1).name());
    dv1.GetTypeInfo();
    printf("%s derivado 1 nao virtual\n",__PRETTY_FUNCTION__);
    der_1 d1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(d1).name());
    d1.GetTypeInfo();

    printf("\n");
    printf("%s derivado 2 virtual\n",__PRETTY_FUNCTION__);
    der_virtual_2 dv2;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(dv2).name());
    dv2.GetTypeInfo();
    printf("%s derivado 2 nao virtual\n",__PRETTY_FUNCTION__);
    der_2 d2;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(d2).name());
    d2.GetTypeInfo();

    base_virtual *bvpointer;
    base *bpointer;

    printf("\n\n%s Alocacao explicita\n",__PRETTY_FUNCTION__);
    printf("\n");
    printf("%s base virtual\n", __PRETTY_FUNCTION__);
    bvpointer = new base_virtual;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s base nao virtual\n", __PRETTY_FUNCTION__);
    bpointer = new base;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 1 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new der_virtual_1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 1 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new der_1;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 2 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new der_virtual_2;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 2 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new der_2;
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;


    printf("\n\n%s Alocacao explicita com typeof tipo\n",__PRETTY_FUNCTION__);
    printf("\n");
    printf("%s base virtual\n", __PRETTY_FUNCTION__);
    bvpointer = new typeof (base_virtual);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s base nao virtual\n", __PRETTY_FUNCTION__);
    bpointer = new typeof (base);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 1 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new typeof (der_virtual_1);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 1 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new typeof (der_1);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 2 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new typeof (der_virtual_2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 2 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new typeof (der_2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n\n%s Alocacao explicita com typeof pointer\n",__PRETTY_FUNCTION__);
    base_virtual *bvpointer2;
    base *bpointer2;
    bvpointer2=new base_virtual;
    bpointer2 = new base;
    printf("\n");
    printf("%s base virtual\n", __PRETTY_FUNCTION__);
    bvpointer = new typeof (*bvpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s base nao virtual\n", __PRETTY_FUNCTION__);
    bpointer = new typeof (*bpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;
    delete bvpointer2;
    delete bpointer2;

    bvpointer2=new der_virtual_1;
    bpointer2 = new der_1;
    printf("\n");
    printf("%s derivado 1 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new typeof (*bvpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 1 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new typeof (*bpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;
    delete bvpointer2;
    delete bpointer2;

    bvpointer2=new der_virtual_2;
    bpointer2 = new der_2;
    printf("\n");
    printf("%s derivado 2 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = new typeof (*bvpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 2 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = new typeof (*bpointer2);
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;
    delete bvpointer2;
    delete bpointer2;


    printf("\n\n%s Alocacao implicita com funcao\n",__PRETTY_FUNCTION__);
    printf("\n");
    printf("%s base virtual\n", __PRETTY_FUNCTION__);
    bvpointer = bv1.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s base nao virtual\n", __PRETTY_FUNCTION__);
    bpointer = b1.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 1 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = dv1.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 1 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = d1.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;

    printf("\n");
    printf("%s derivado 2 virtual\n",__PRETTY_FUNCTION__);
    bvpointer = dv2.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bvpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bvpointer).name());
    bvpointer->GetTypeInfo();
    delete bvpointer;
    printf("%s derivado 2 nao virtual\n",__PRETTY_FUNCTION__);
    bpointer = d2.Alloc();
    printf ("%s typeid %s\n", __PRETTY_FUNCTION__, typeid(bpointer).name());
    printf ("%s typeid * %s\n", __PRETTY_FUNCTION__, typeid(*bpointer).name());
    bpointer->GetTypeInfo();
    delete bpointer;
    return 0;
    }

观察结果后，一种可能的解决方案是让每个派生类都有自己的分配器。我不明白。如果基类有分配器，为什么派生类仍然使用基类的 typeof，而不是派生类的？

Answer 1

通常的方法是提供一个(协变？)clone() 函数:

struct Base
{
    virtual Base * clone()  { return new Base(*this); }
    virtual ~Base() { }
};

struct Derived : Base
{
    virtual Derived * clone() { return new Derived(*this); }   // this is an override
};

然后你可以说:

Base * p = get();
Base * q = p->clone();