C ++实现抽象类

Question

I am trying to write a derived class called TerminalPlayer that inheritances a class Player given the declaration of virtual const Card playCard(const Card opponentCard) = 0; how would you implement the inherited playCard in the abstract class and what does the = 0 at the end of the prototype mean?

I also have the error in the main testing code that gives the error: cannot allocate an object of abstract type 'Player'. I think it is because I am not implementing the Player class correctly but I don't know how to fix it.

Player.h

    #ifndef PLAYER_H_
    #define PLAYER_H_

    #include <vector>
    #include "Card.h"

    #define MAX_HAND_SIZE 3

    // Abstract Player classS
    class Player {

        public:
            // Deconstructor
            virtual ~Player() {
            }

            // Play a card. If the player receives a joker then this player is going first
            virtual const Card playCard(const Card opponentCard) = 0;

            // Receive a card from the dealer
            void receiveCard(const Card c) {
                hand.push_back(c);
            }

            // Add points to the score
            void addScore(unsigned s) {
                score += s;
            }

            // Get the score
            int getScore() const {
                return score;
            }

            // Return true if the player has cards in the hand
            bool hasCards() const {
                return (hand.size() != 0);
            }

            // Receive the cards played from the previous round. This member function would be used by a computer player that may need to 'see' what cards were played.
            void cardsPlayed(const Card card1, const Card card2) {

            }

            // Output the players name
            friend std::ostream& operator <<(std::ostream& out, const Player& p);

        protected:
            // Constructor. Since this is an abstract class we do not want anyone instantiating a player class so we make it protected.
            Player(std::string name) :
                    score(0), name(name), hand(0) {
            }

            int score;
            std::string name;
            std::vector<Card> hand;
    };

    #endif

TerminalPlayer.h

    #ifndef TERMINALPLAYER_H_
    #define TERMINALPLAYER_H_

    #include "Player.h"

    class TerminalPlayer : public Player {
    public:
        TerminalPlayer(std::string name);
        virtual ~TerminalPlayer();
    };

    #endif

TerminalPlayer.cpp

    #include "Player.h"
    Card playCard(const Card opponnentCard){
        // TODO: playCard code here
    }

Test.cpp

    int main(){

        // This initialization give error: cannot allocate an object of abstract type ‘Player’
        TerminalPlayer player1 = Player("Player1");

        return 0;
    }

Answer 1

The = 0' means that this is a pure virtual function. This type of function must be defined by any class which inherits from the base class AND is instantiated within a program.

Since your base class declares:

// Play a card. If the player receives a joker then this player is going first
virtual const Card playCard(const Card opponentCard) = 0;

You should implement this function within your derived class. You come close in TerminalPlayer.cpp:

const Card TerminalPlayer::playCard(const Card opponnentCard){
    // TODO: playCard code here
}

What you are missing the the TerminalPlayer:: scoping shown above. Also missing is the function declaration in the derived class. You need to add:

virtual const Card playCard(const Card opponentCard) override;

Within the class TerminalPlayer . Put it right after the destructor.

That should do it.

One thought: the const qualifier on the return value is not necessary since you are returning by value.