cpp学习笔记2--class

 今天继续学习类

class BankAccount {
private:
    double balance;
protected:
    std::string accountNumber;
public:
    void deposit(double amount) {
        balance += amount;
    }
    double getBalance() const {
        return balance;
    }
    void set(double b,std::string x)
    {
        balance = b;
        accountNumber = x;
    }
    void print() const
    {
        std::cout << "balance:" << balance << std::endl;
        std::cout << "account:" << accountNumber << std::endl;
    }
};
// 派生类
class SavingsAccount : public BankAccount {
private:
    double interestRate;
public:
    // 新增功能
    void applyInterest() {
        deposit(getBalance() * interestRate);
    }

    // 可以访问基类的protected成员
    void setAccountNumber(const std::string& num) {
        accountNumber = num; // 允许访问基类的protected成员
    }

    // 不能直接访问基类的private成员
    // void setBalance(double b) {
    //     balance = b; // 错误！不能访问基类的private成员
    // }
};

        double getBalance() const，首先这个const放在后面我是没见过的，它的意思是这个函数不允许修改除了mutable型之外的class的成员变量，无论是private还是protected。

        protected有点像介于public和private之间，像private是因为BankAccount a1;a1.accountNumber是无法访问的，而同一类的public函数可以访问；像public是因为，派生类SavingsAccount可以直接访问pubilc和protected，却不能访问private。

        接下来是派生类，它可以直接继承基类的public和protected，但是不会把值也继承了。如下的输出结果，还是要自己初始化的。需要注意的是，s1虽然无法访问balance，但是可以访问print，print可以访问balance，也就是private变量不可以直接访问，但可以间接访问。

balance:1.5
account:Alice
balance:0
account:
balance:3.1
account:Mike

BankAccount a1;
a1.set(1.5, "Alice");
a1.print();
SavingsAccount s1;
s1.print();
s1.set(3.1, "Mike");
s1.print();

关于类中定义静态变量

class SavingsAccount : public BankAccount {
private:
    double interestRate;
    static double bank;
    double road;
public:

    void s_print()
    {
        std::cout << "interest:" << &interestRate << std::endl;
        std::cout << "bank:" << &bank << std::endl;
        std::cout << "road:" << &road << std::endl;
    }
};

static变量在main.cpp中必须初始化，而且必须在main外，因为static是在存储区的，不在堆栈区。

下面的输出就是

interest:000000141DEFF820
bank:00007FF634185078
road:000000141DEFF828
s1:000000141DEFF7F8
cat:00007FF634185680

可以看到，bank的地址和interest、road乃至s1都不一样，只和cat是一类的，说明它不属于堆栈区。既然如此，那修改一个bank所有SavingsAccount类的bank值应该都会改变。

double SavingsAccount::bank = 0.0;
double cat;
int main() {
    SavingsAccount s1;
    s1.s_print();
    std::cout << "s1:" << &s1 << std::endl;
    std::cout << "cat:" << &cat << std::endl;
    return 0;
}

再验证一下。

void set_bank(double x)
{
    bank = x;
}

void s_print()
{
    std::cout << "interest:" << &interestRate << std::endl;
    std::cout << "bank:" << &bank << std::endl;
    std::cout << "road:" << &road << std::endl;
    std::cout << "bank value:" << bank << std::endl;
}

SavingsAccount s1,s2;
s1.s_print();
std::cout << "s1:" << &s1 << std::endl;
std::cout << "cat:" << &cat << std::endl;
s2.s_print();
s1.set_bank(6.2);
s1.s_print();
s2.s_print();

输出

interest:000000141DEFF820
bank:00007FF634185078
road:000000141DEFF828
bank value:10
s1:000000141DEFF7F8
cat:00007FF634185680
interest:000000141DEFF7E8
bank:00007FF634185078
road:000000141DEFF7F0
bank value:10
interest:000000141DEFF820
bank:00007FF634185078
road:000000141DEFF828
bank value:6.2
interest:000000141DEFF7E8
bank:00007FF634185078
road:000000141DEFF7F0
bank value:6.2

确实是这样的