boost::static_pointer_cast、boost::dynamic_pointer_cast和boost::const_pointer_cast

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当我们用“裸”指针进行类层次上的上下行转换时，可以使用dynamic_cast。当然我们也可以使用static_cast，只是dynamic_cast在进行下行转换的时候（即基类到派生类）具有类型检查功能，而static_cast没有。因此存在安全问题。

当我们使用智能指针时，如果需要进行类层次上的上下行转换时，可以使用boost::static_pointer_cast和boost::dynamic_pointer_cast。（C++11中也支持智能指针和转换，只是命名空间改成std即可）。

1. static_pointer_cast

#include <iostream>#include <boost/shared_ptr.hpp>#include <boost/make_shared.hpp>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）boost::shared_ptr<CDerive> spDeriveUp;boost::shared_ptr<CBase> spBaseUp;spDeriveUp = boost::make_shared<CDerive>();spBaseUp = boost::static_pointer_cast<CBase>(spDeriveUp);spBaseUp->myBase();//下行的转换（基类到派生类的转换）这是不安全的boost::shared_ptr<CDerive> spDeriveDown;boost::shared_ptr<CBase> spBaseDown;spBaseDown = boost::make_shared<CBase>();spDeriveDown = boost::static_pointer_cast<CDerive>(spBaseDown);spDeriveDown->myBase();//不允许访问myDerivereturn 0;}

2. dynamic_pointer_cast

#include <iostream>#include <boost/shared_ptr.hpp>#include <boost/make_shared.hpp>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）boost::shared_ptr<CDerive> spDeriveUp;boost::shared_ptr<CBase> spBaseUp;spDeriveUp = boost::make_shared<CDerive>();spBaseUp = boost::dynamic_pointer_cast<CBase>(spDeriveUp);spBaseUp->myBase();//下行的转换（基类到派生类的转换)boost::shared_ptr<CDerive> spDeriveDown;boost::shared_ptr<CBase> spBaseDown;spBaseDown = boost::make_shared<CBase>();spDeriveDown = boost::dynamic_pointer_cast<CDerive>(spBaseDown);if (spDeriveDown == NULL)//由于会进行类型的检查，这边是返回NULLcout << "spDeriveDown is null" << endl;return 0;}

3. const_pointer_cast

#include <iostream>#include <boost/shared_ptr.hpp>#include <boost/make_shared.hpp>using namespace std;int main(void){{boost::shared_ptr<int> spInt;boost::shared_ptr<const int> spConstInt;spInt = boost::make_shared<int>(100);spConstInt = boost::const_pointer_cast<const int>(spInt);cout << "*spConstInt = " << *spConstInt << endl;}{boost::shared_ptr<int> spInt;boost::shared_ptr<const int> spConstInt;//事实上不允许通过spInt修改值spConstInt = boost::make_shared<const int>(100);spInt = boost::const_pointer_cast<int>(spConstInt);cout << "*spInt = " << *spInt << endl;}return 0;}

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