C++智能指針類型轉換

#include <iostream>
#include <memory>
 
struct Base 
{ 
    int a; 
    virtual void f() const { std::cout << "I am base!\n";}
    virtual ~Base(){}
};
 
struct Derived : Base
{
    void d()
    { std::cout << "Derived:d()\n"; }
    void f() const override
    { std::cout << "I am derived!\n"; }
    ~Derived(){}
};
 
int main(){
    auto basePtr = std::make_shared<Base>();
    std::cout << "Base pointer says: ";
    basePtr->f();
 
    auto derivedPtr = std::make_shared<Derived>();
    std::cout << "Derived pointer says: ";
    derivedPtr->f();
    
    
    // 上行的轉換（派生類到基類的轉換）
    std::cout << "Derived pointer static_pointer_cast to Base says: ";
    std::static_pointer_cast<Base>(derivedPtr)->f();


    // 直接轉型，不構造臨時 shared_ptr
    // 下行的轉換（基類到派生類的轉換)
    std::cout << "Base pointer static_cast to derived says: ";
    static_cast<Derived*>(basePtr.get())->d();

    // 構造臨時 shared_ptr ，而後調用 operator->
    // 下行的轉換（基類到派生類的轉換)
    std::cout << "Base pointer static_pointer_cast to derived says: ";
    std::static_pointer_cast<Derived>(basePtr)->d();

    // 轉換成對象
    // 下行的轉換（基類到派生類的轉換)
    Derived& derived = static_cast<Derived&>(*basePtr);
    std::cout << "Base pointer static_cast to derived Obeject says: ";
    derived.d();

    // std::cout << "Base pointer dynamic_cast to derived says: ";
    // dynamic_cast<Derived*>(basePtr.get())->f();  //段錯誤

    // Derived& derived1 = dynamic_cast<Derived&>(*basePtr);
    // derived1.d();       // 異常

    // 下行的轉換（基類到派生類的轉換)
    auto downcastedPtr = std::dynamic_pointer_cast<Derived>(basePtr);
    if(downcastedPtr)
    { 
        std::cout << "Base pointer dynamic_pointer_cast to derived says: ";
        downcastedPtr->d(); 
    }
 
    // All pointers to derived share ownership
    std::cout << "Pointers to underlying derived: " 
            << derivedPtr.use_count() 
            << "\n"; 
}