c++中dynamic_pointer_cast强制类型转换

#include <iostream>
#include <memory>
 
using namespace std;
 
class CBase
{
public:
    CBase() { }
    virtual ~CBase() { }
 
    void myBase()
    {
        cout << "CBase::myBase" << endl;
    }
};
 
class CDerive : public CBase
{
public:
    CDerive() { }
    ~CDerive() { }
 
    void myDerive()
    {
        cout << "CDerive::myDerive" << endl;
    }
};
 
int main(void)
{
    //上行的转换（派生类到基类的转换）
    shared_ptr<CDerive> spDeriveUp;
    shared_ptr<CBase> spBaseUp;
    spDeriveUp = make_shared<CDerive>();
    spBaseUp = dynamic_pointer_cast<CBase>(spDeriveUp);
    spBaseUp->myBase();
 
    //下行的转换（基类到派生类的转换)
    shared_ptr<CDerive> spDeriveDown;
    shared_ptr<CBase> spBaseDown;
    spBaseDown = make_shared<CBase>();
    spDeriveDown = dynamic_pointer_cast<CDerive>(spBaseDown);
    if (spDeriveDown == NULL)    //由于会进行类型的检查，这边是返回NULL
        cout << "spDeriveDown is null" << endl;
 
    /*
    shared_ptr<CDerive> spDeriveDown;
    shared_ptr<CBase> spBaseDown;
    spBaseDown = make_shared<CDerive>();
    spDeriveDown = dynamic_pointer_cast<CDerive>(spBaseDown);
    if (spDeriveDown == NULL)    //由于会进行类型的检查，这边是返回NULL
        cout << "spDeriveDown is null" << endl;
    spDeriveDown->myDerive();
    */
 
    return 0;

}

运行结果：

#include <iostream>
#include <memory>
 
using namespace std;
 
class CBase
{
public:
    CBase() { }
    virtual ~CBase() { }
 
    void myBase()
    {
        cout << "CBase::myBase" << endl;
    }
};
 
class CDerive : public CBase
{
public:
    CDerive() { }
    ~CDerive() { }
 
    void myDerive()
    {
        cout << "CDerive::myDerive" << endl;
    }
};
 
int main(void)
{
    //上行的转换（派生类到基类的转换）
    shared_ptr<CDerive> spDeriveUp;
    shared_ptr<CBase> spBaseUp;
    spDeriveUp = make_shared<CDerive>();
    spBaseUp = dynamic_pointer_cast<CBase>(spDeriveUp);
    spBaseUp->myBase();
 
    //下行的转换（基类到派生类的转换)
    /*
    shared_ptr<CDerive> spDeriveDown;
    shared_ptr<CBase> spBaseDown;
    spBaseDown = make_shared<CBase>();
    spDeriveDown = dynamic_pointer_cast<CDerive>(spBaseDown);
    if (spDeriveDown == NULL)    //由于会进行类型的检查，这边是返回NULL
        cout << "spDeriveDown is null" << endl;
    */
 
    shared_ptr<CDerive> spDeriveDown;
    shared_ptr<CBase> spBaseDown;
    spBaseDown = make_shared<CDerive>();
    spDeriveDown = dynamic_pointer_cast<CDerive>(spBaseDown);
    if (spDeriveDown == NULL)    //由于会进行类型的检查，这边是返回NULL
        cout << "spDeriveDown is null" << endl;
    spDeriveDown->myDerive();    
 
    return 0;

}

运行结果：

 一些结论：

 　　子类向基类转是一定没有问题的。基类转子类的话，必须基类的动态类型与要转换的子类一致。