C++ 无法将 base class "this" 指针分配给 base class 构造函数中的派生 class 对象

Question

您好，我正在学习 C++，并且正在尝试构造一些东西，其中使用以下构造将 base class *this 指针分配给派生的 class 对象。我的问题是，这种方式在 C++ 中是否可行？因为，当我尝试编译它时，出现错误

错误：“Derived_1”之前需要类型说明符

如果这样的事情是可能的，那么正确的方法是什么。我的 objective 这样做是通过基于 DISPLAY 类型的基础 class 自动获取相对派生的 class 实现，以避免 main 中的 if-else 类子句。任何帮助将不胜感激。

#ifndef _MAIN_HPP_
#define _MAIN_HPP_

#include <iostream>

enum class DISPLAY {
    DERIVED_1 = 1,
    DERIVED_2 = 2
};

class Base {
    private:
        int variable = 0;
    public:
        Base(){std::cout<<"Base Class Empty Constructor"<<std::endl;}
        Base(DISPLAY Type) {
            std::cout<<"Base Class Constructor Derived_"<<static_cast<int>(Type)<<std::endl;
            switch(Type) 
            {
                case(DISPLAY::DERIVED_1): {
                    *this = new Derived_1();
                }break;
                case(DISPLAY::DERIVED_2): {
                    *this = new Derived_2();
                }break;
            }
        }
        virtual ~Base() {std::cout<<"This is Base Class Destructor"<<std::endl;}
        virtual int Sum(int x, int y) {return x+y;};
};

class Derived_1 : public Base {
    public:
        Derived_1(){std::cout<<"This is Derived_1 Class Constructor"<<std::endl;}
        virtual ~Derived_1() {std::cout<<"This is Derived_1 Class Destructor"<<std::endl;}
        int Sum(int x, int y) {return (x*2)+(y*3);}
};

class Derived_2 : public Base {
    public:
        Derived_2(){std::cout<<"This is Derived_2 Class Constructor"<<std::endl;}
        virtual ~Derived_2() {std::cout<<"This is Derived_2 Class Destructor"<<std::endl;}
        int Sum(int x, int y) {return (x*1)+(y*2);}
};
#endif

#include "main.hpp"

using namespace std;

int main(int argc, char **argv)
{
    Base *base = new Base(DISPLAY::DERIVED_1);
    cout<<"SUM:"<<base->Sum(2, 10)<<endl;
    return 0;
}

错误摘要：

/home/junaid/workspace/Learn_Cpp/main.hpp: In constructor ‘Base::Base(DISPLAY)’:
/home/junaid/workspace/Learn_Cpp/main.hpp:21:33: error: expected type-specifier before ‘Derived_1’
   21 |                     *this = new Derived_1();
      |                                 ^~~~~~~~~
/home/junaid/workspace/Learn_Cpp/main.hpp:24:33: error: expected type-specifier before ‘Derived_2’
   24 |                     *this = new Derived_2();
      |                                 ^~~~~~~~~

Build finished with error(s).
The terminal process terminated with exit code: -1.

Answer 1

你绝对不能做你正在做的事。你不能这样做：

                *this = new Derived_1();

我什至不想去想这其中的所有错误。相反，您想要的是实现工厂模式（您可以 google 实现）。

C++ 无法将 base class "this" 指针分配给 base class 构造函数中的派生 class 对象

C++ Unable to assign base class "this" pointer the derived class object inside base class constructor

c++

inheritance

pointers

this