C++如何使用派生类构造函数销毁基类中的对象

Question

So in my program I have a derived class called hangman and a derived class called HangmanGraphics. The issue I'm having is after the game I want to reset my variables but for some reason my base constructor does not get restarted or even called. When I run the program it will go to the base constructor once but towards the end of the program when:

#include "player.h"
#include "hangman.h"
#include "HangmanGraphics.h"

using namespace std;

int main()
{

    HangmanGraphics game2;
    while(1)
    {
        game2.Play();

        if(game2.Play()=='Y')
        {
            game2=HangmanGraphics();

            //in this part of the code I want to reset my base
            //constructor values but how do I do that by using the 
            //derived construtor  

            continue;
        }
    }
}

Answer 1

Cheap hack. Moving the definition of game2 into the while loop will provide a new game2 every pass though the loop:

int main()
{
    while(1)
    {
        HangmanGraphics game2; // game2 created here
        // do stuff
    } // game2 destroyed here
}

There is not enough information provided in your question to say whether this is a good thing to do or a bad. But...

game2=HangmanGraphics();

will do five things:

1. Call HangmanGraphics's constructor to create a temporary HangmanGraphics.
2. This results in a call to Hangman's constructor
3. Call HangmanGraphics's = operator. If one has not been specified, the default will be to copy the contents of the right hand HangmanGraphics into the left. It will copy a pointer, but not the pointed-to data.
4. Call the destructor on the temporary HangmanGraphics
5. Which results in a call to Hangman's destructor

If you have any dynamically allocated storage in HangmanGraphics or Hangman, you need to read up on the Rule of Three

Anything dynamically allocated within the temporary HangmanGraphics and it's base Hangman will be (or should have been) deleted by the destructor. If a operator= was not custom-defined to handle the dynamic data, game2 now contains pointers to invalid storage.

Example:

class base
{
public:
    base(): baseval(number++), interestingData(new int(number++))
    {
        cout << "base ctor " << baseval << "," << * interestingData <<endl;
    }
    virtual ~base()
    {
        cout << "base dtor " << baseval << "," << * interestingData <<endl;
        delete interestingData;
    }
private:
    int baseval;
    int * interestingData;
};

class derived: public base
{
public:
    derived():base()
    {
        cout << "derived ctor" <<endl;
    }
    virtual ~derived()
    {
        cout << "derived dtor" <<endl;
    }
    derived & operator=(const derived & rhs) //only exists to do the cout
    {
        cout << "derived =" <<endl;
        base::operator=(rhs);
        return *this;
    }
};

int main()
{
    derived test;
    cout << "created test" << endl;
    test = derived();
    cout << "exit" << endl;
}

Output

base ctor 0,1
derived ctor
created test
base ctor 2,3
derived ctor
derived =
derived dtor
base dtor 2,3
exit
derived dtor
base dtor 2,4067440

interestingdata in the last print is smashed because the address it points to has been reassigned. Even more fun, it is about to be deleted a second time and that should crash your program.