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Question

我正在用一个简单的AI编写一个简单的tictactoe游戏。我在初始化游戏对象时遇到错误。我的游戏对象使用名为setPlayerNum的方法初始化了两个玩家对象，每个对象的玩家编号属性都设置为“一个”或“两个”。尽管我从不使用“ this”关键字，但是却收到一条错误消息，指出“ this is nullptr”以及写访问冲突。

我尝试过取消引用播放器对象，以及将播放器编号属性设置为公共变量（以防出现范围错误）。都不适合我。

int main() {

    game g;
    g.printTitleCard();
    std::system("pause");

    return 0;
}

#include "board.h"
#include "player.h"

class game {

public:
    game();
    ~game();
    void printTitleCard();
    void play();
    int victory();
    void victoryScreen(int winner);

private:
    board * gameBoard;
    player * player_one;
    player * player_two;
    int turns;
    int maxTurns = 9;
};

game::game() {
    gameBoard = new board();
    player_one->setPlayerNum(1);
    player_two->setPlayerNum(2);
}

class player {

public:
    player();
    ~player();
    void setPlayerNum(int);
    int getPlayerNum();
    void updateScore(int);
    int retrieveWins();
    int retrieveLoses();
    int retrieveTies();


private:
    int playerNum = 0;
    int wins = 0;
    int loses = 0;
    int ties = 0;
};

void player::setPlayerNum(int num) {
    playerNum = num;
}

Answer 1

game::game() {
    gameBoard = new board();
    player_one->setPlayerNum(1);
    player_two->setPlayerNum(2);
}

不会在调用player之前实例化任何setPlayerNum，这意味着没有player的有效实例可以调用setPlayerNum。之后，程序进入undefined behaviour，所有投注都关闭了。

解决方案：

解决此问题的最佳方法是回到

class game {

public:
    game();
    ~game();
    void printTitleCard();
    void play();
    int victory();
    void victoryScreen(int winner);

private:
    board * gameBoard;
    player * player_one;
    player * player_two;
    int turns;
    int maxTurns = 9;
};

并且不要对gameBoard，player_one和player_two使用指针。

    board * gameBoard;
    player * player_one;
    player * player_two;

成为

    board gameBoard;
    player player_one;
    player player_two;

和

game::game() {
    gameBoard = new board();
    player_one->setPlayerNum(1);
    player_two->setPlayerNum(2);
}

成为

game::game() {
    player_one.setPlayerNum(1);
    player_two.setPlayerNum(2);
}

仅在绝对必要时使用new，在几乎没有库容器和智能指针的世界中。这里的更多信息：Why should C++ programmers minimize use of 'new'?

不相关：

如果更改播放器构造函数以将播放器编号作为参数，则setPlayerNum可能变得不必要，而game的构造函数可能会变成

game::game(): player_one(1), player_two(2){
}

附录

有什么办法可以将它们保留为指针？

是的，但这不是一个好主意，因为您承担了很多额外的责任。在使用球员之前，您天真地new个球员。您必须delete个播放器和game中的析构器，并且您必须编写自己的副本（如果需要，请移动）构造函数和赋值（并移动赋值）运算符，以确保对象已正确复制和分配（有关为何需要此材料的更多信息，请参见What is The Rule of Three?）。

game::game() {
    gameBoard = new board();
    player_one = new player; //added to get player instance
    player_two= new player; // added to get player instance

    player_one->setPlayerNum(1);
    player_two->setPlayerNum(2);
}

// new function: copy constructor
game::game(const game & source) {
    gameBoard = new board(*source.gameBoard);
    player_one = new player(*source.player_one);
    player_two= new player(*source.player_two);
    turns = source.turns;
    maxTurns = source.maxTurns; 
}
game::~game() {
    delete gameBoard; // you needed to have this if you didn't already
    delete player_one; // added to release player instance
    delete player_two; //added to release player instance
}
// new function swap function (used by assignment operator)
game::swap(game & source) {
    std::swap(gameBoard, source.gameBoard);
    std::swap(player_one , source.player_one);
    std::swap(player_two, source.player_two);
    std::swap(turns, source.turns);
    std::swap(maxTurns, source.maxTurns);
}
// new function assignment operator
game & operator=(game source)
{
    swap(source);
    return * this;
}

查看您必须编写的所有其他内容。可能其中有一个或两个错误。

赋值运算符正在利用Copy and Swap Idiom。它通常不是最快的分配方式，但它很容易编写且很难出错。我从复制和交换开始，一直使用它，直到生活证明它是性能瓶颈。

如果gameBoard，player_one和player_two是自动变量，除非board写得不好，否则编译器将为您生成析构函数和所有复制代码。您的目标之一应该是使own a resource的所有类都遵守Rule of Three or Five，以便包含它们的类可以利用Rule of Zero。

没有调用“ this”时如何解决“ this is nullptr”

1 个答案:

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