c++ - 为什么我的代码出现段错误

Question

对于我来说，我根本无法理解为什么这个程序会出现段错误。问题是它检索 vector 容器中的对象使用菜单类中的函数使用 get_command() 并且由于某种原因在逐行测试主要函数后导致段错误:

menu->get_command()->execute();

我已经尝试更改语法以创建一个新的命令对象来存储从 get_command() 返回的对象并将索引更改为 0 和 -1 之间的值，但仍然没有修复错误。我至少花了几个小时试图找出原因，但似乎找不到解决方案。

class Base {
public:
    /* Constructors */
    Base() { };

    /* Pure Virtual Functions */
    virtual double evaluate() = 0;
    virtual std::string stringify() = 0;
};

class op : public Base
{
public:
    op() { };
    op(double op1) { operand = op1; }

    double evaluate() { return operand; }

    string stringify() {
        string value = to_string(operand);
        return value;
    }
private:
    double operand;
};

class Command {
protected:
    Base* root;

public:
    Command() { this->root = nullptr; }
    double execute() { return root->evaluate(); }
    std::string stringify() { return root->stringify(); }
    Base* get_root() { return root; }
};



class Menu {
private:
    int history_index; // Indexes which command was last executed, accounting for undo and redo functions
    std::vector<Command*> history; // Holds all the commands that have been executed until now

public:
    Menu() {
        // Constructor which initializes the internal members
        history_index = -1;
    }

    std::string execute() {
        // Returns the string converted evaluation of the current command
        return to_string(history[history_index - 1]->execute());
    }

    std::string stringify() {
        // Returns the stringified version of the current command
        return history[history_index]->stringify();
    }

    bool initialized() {
        // Returns if the history has an InitialCommand, which is necessary to start the calculation
        if (history[history_index] != nullptr)
            return true;
        else
            return false;
    }

    void add_command(Command* cmd) {
        // Adds a command to the history (does not execute it), this may require removal of some other commands depending on where history_index is
        history.push_back(cmd);
        history_index++;
    }

    Command* get_command() {
        // Returns the command that the history_index is currently referring to
        return history[history_index];
    }

    void undo() {
        // Move back one command (does not execute it) if there is a command to undo
        history_index--;
    }

    void redo() {
        // Moves forward one command (does not execute it) if there is a command to redo
        history_index++;
    }
};


class InitialCommand : public Command {
protected:
    Base* root;

public:
    InitialCommand(Base* b) { this->root = b; }
    double execute() { return root->evaluate(); }
    std::string stringify() { return root->stringify(); }
    Base* get_root() { return root; }
};


void main()
{
    Menu* menu = new Menu();
    InitialCommand* temp = new InitialCommand(new op(7));
    menu->add_command(temp);
    EXPECT_EQ(menu->get_command()->execute(), 7);

    system("PAUSE");

}

Answer 1

您没有正确执行继承，因为您在 Command 和 InitialCommand 之间复制导致错误的字段。

两个命令类都有一个 Base *root 成员和非虚拟的 execute 方法。当您构造一个新的 InitialCommand 对象时，InitialCommand::root 对象指向为它创建的 op，而 Command::root 保持 NULL 因为 Command 的默认构造函数。然后，当您调用 menu->get_command() 时，它会调用 Command::execute 因为 execute 是非虚拟的并且 menu 是一个 Command *。 Command::execute 将取消引用 NULL root，导致您的段错误。

从 InitialCommand 中移除 Base *root 成员，并将参数传递给 Command 中的构造函数。您可能希望将一些方法(例如 execute)设为虚拟。