我知道dynamic_cast，但我真的想知道什么时候使用它？为什么不总是将适当的函数添加到基类中，并且总是可以访问所需的函数？
这是否有任何性能方面？
我写了一个小例子来说明这一点。如果基类A中没有纯虚函数myUniqueFunc（），则只能使用dynamic_cast调用派生类中的函数。所以我总是把这个函数包含在基类中…
此外，dynamic_cast需要大量开销，因此需要大量计算时间。

#include <iostream>
#include <vector>

class A
{
public:
    virtual void printSomething() { std::cout << "class A: hello." << std::endl; };
    virtual void myUniqueFunc() = 0 ;
    virtual ~A() {};
};

class B : public A
{
public:
    void printSomething() override { std::cout << "class B: hello." << std::endl; };
    void myUniqueFunc() { std::cout << "class B: Only I can do this..." << std::endl; }
    B() {};
    virtual ~B() {};

};

class C : public A
{
public:
    //void printSomething() override { std::cout << "class C: hello." << std::endl; };
    void myUniqueFunc() { std::cout << "class C: Only I can do this..." << std::endl; }
    C() {};
    ~C() {};
};

int main()
{
    std::vector<std::unique_ptr<A>> myObjects;
    myObjects.emplace_back(std::make_unique<B>());
    myObjects.emplace_back(std::make_unique<C>());
    myObjects.emplace_back(std::make_unique<B>());
    myObjects.emplace_back(std::make_unique<C>());
    myObjects.emplace_back(std::make_unique<B>());
    myObjects.emplace_back(std::make_unique<C>());

    // 1. fast solution via complete virtual functions in base class
    // iterate over all Objects (common functions only
    for (auto& curObjects : myObjects)
    {
        curObjects->printSomething();
        curObjects->myUniqueFunc();
    }

    
    // 2. slow solution via incomplete virtual functions in base class and downcast to derived classes
    // iterate over all Objects (and calls the special func from the derived classes only)
    for (auto& curObjects : myObjects)
    {
        // downcast to the derived classes to get the unique functions
        if (typeid(curObjects) == typeid(B))
        {
            B& aBClass = dynamic_cast<B&>(*curObjects);
            aBClass.myUniqueFunc();
        }
            
        if (typeid(curObjects) == typeid(C))
        {
            // downcast to the derived classes to get the unique functions
            C& aCClass = dynamic_cast<C&>(*curObjects);
            aCClass.myUniqueFunc();
        }
        
    }

    return 0;
}