C++ 实现抽象类

Question

我正在尝试编写一个名为 TerminalPlayer 的派生 class，它继承了一个 class Player，给定虚拟 const Card playCard(const Card opponentCard) = 0 的声明；你将如何在抽象中实现继承的 playCard class 以及原型末尾的 = 0 是什么意思？

我在给出错误的主要测试代码中也有错误：无法分配抽象类型“Player”的对象。我认为这是因为我没有正确实现播放器 class 但我不知道如何修复它。

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

航站楼Player.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

= 0'表示这是一个pure virtual函数。这种类型的函数必须由继承自基础 class 并在程序中实例化的任何 class 定义。

由于您的基地 class 声明：

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

您应该在派生的 class 中实现此功能。你接近 TerminalPlayer.cpp:

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

您缺少上面显示的 TerminalPlayer:: 范围。还缺少派生 class 中的函数声明。您需要添加：

virtual const Card playCard(const Card opponentCard) override;

内 class TerminalPlayer。把它放在析构函数之后。

应该可以了。

一个想法：return 值上的 const 限定符不是必需的，因为您是按值 returning。

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