Question

衍生演员阵容的真实可接受使用范例是什么？我一直以为它们只在实施“黑客”时使用，但如果不是这样，那么有人可以给出一个可以接受的例子吗？

Answer 1

@ user997112

[在底部编辑]

您好。下面我们使用一组具有共同祖先的随机多态指针通过共同的界面使用特定派生类之一进行额外的工作我们需要dynamic_cast或typeid来了解这一点.... 主要功能有呼叫
然后是阶级声明
然后动态演员表结束了删除用new创建的对象未显示

#include <iostream>
#include <algorithm>
#include <random>
#include <exception>
using namespace std;

int dynamic_test();
int main()
{
    cout << "Hello world!" << endl;
    dynamic_test();
    return 0;
}

............

class basex {
    public:
    virtual ~basex() {};
    virtual void work() const = 0;
};
class next1x : public basex {
    public:
    void work() const override {cout << "1";/*secret*/}
};
class next2x : public basex {
    public:
    void work() const override {cout << "2";/*secret*/}
};
class next3x : public basex {
    public:
    void work() const override {cout << "3";/*secret*/}
};

std::vector<basex *> secret_class_picker()
{
    //pick classes with common base at random
    std::random_device rd;
    std::uniform_int_distribution<int>  ud(1,3);
    std::mt19937 mt(rd());
    std::vector<int> random_v;
    for (int i = 0; i < 22; ++i)
        random_v.push_back( ud(mt) );
    cout << "Random" << endl;
    for ( auto bq : random_v) //inspecting for human reader
        cout << bq << " ";
    std::vector<basex *> v;
    basex * bptr;
    for (auto bq : random_v) {
            switch(bq)
            {

            default: throw std::exception(); break;
            case 1: bptr = new next1x; break;
            case 2: bptr = new next2x; break;
            case 3: bptr = new next3x; break;
            }

            v.push_back(bptr);
    }
    cout << "Objects Created " << v.size() << endl;
    return v;

}

//this function demands a more derived class
int special_work(const next3x *)
{
    //elided
    cout <<"[!]";
    return 0;
}

int dynamic_test()
{
    std::vector<basex *> v = secret_class_picker();//delete these pointer later
    cout <<"Working with random polymorphic pointers"<<endl;
    for (const auto bq : v)
    {
        bq->work();//polymorphic
        next3x * ptr = dynamic_cast<next3x *>(bq);
        if (nullptr != ptr) special_work(ptr); //reserved for particular type
    }
    return 0;
}

...................... alternative

int dynamic_static_typeid()
{
    std::vector<basex *> v = secret_class_picker();
    cout <<"Working with random polymorphic pointers"<<endl;
    int k(0);
    for (const auto bq : v)
    {
        bool flipflop = (k % 2) == 0;
        bq->work();//polymorphic
        //cout << "[*]"<< typeid(*bq).name();//dereference

        if (flipflop) {
            next3x * dc_ptr = dynamic_cast<next3x *>(bq);//not constant time in general
            if (nullptr != dc_ptr) {
                special_work(dc_ptr); //reserved for particular type
                ++k;
            }
        }
        else {
            if (typeid(next3x) == typeid(*bq)){//constant time
                auto sc_ptr = static_cast<next3x *>(bq);//constant time
                special_work(sc_ptr);
                ++k; cout <<"[sc]";
            }
        }
        cout << endl;
    }
    return 0;
}

何时使用dynamic_cast的可接受示例？

1 个答案: