关于继承的困惑

当我遇到这个问题时，我正在测试一个关于分层继承的小程序。该程序包含一个父类银行和两个子类提款和存款。

#include<iostream>
#include<conio.h>
#include<stdlib.h>
//Hierarchical Inheritance
using namespace std;
class bank{
protected:
char name[20];
int age, id;
float bal;

public:
void getData(){
cout <<"Enter your name, age, bank ID and balance" << endl;
cin >> name >> age >> id >> bal;
}
void display(){
cout <<"Name: " << name << endl << "ID: " << id << endl;
cout <<"Age: " << age <<endl <<"Balance: " << bal << endl;
}
void check(){
cout <<"Your balance is " << bal << endl;
}
};
class withdraw : public bank{
float wd;
public:
void withdrawMoney(){
cout << "Enter the amount to withdraw" << endl;
cin >> wd;
if(wd > bal)
cout << "You cannot withdraw that much. Your balance is only " << bal << endl;
else{
bal = bal-wd;
cout << "You successfully withdrew " << wd << ". Your remaining balance is " << bal << endl;
}
}
};
class deposit : public bank{
float dp;
public:
void depo(){
cout <<"Enter the amount to deposit" << endl;
cin >> dp;
bal = bal+dp;
cout <<"You successfully deposited " << dp << ". Your balance is now " << bal << "." << endl;
}
};

int main()
{
int c;
bank b;
deposit d;
withdraw w;
b.getData();
do{
cout <<"***The Dank Bank***" << endl;
cout <<"What do you want to do?n 1)Withdrawn 2)Depositn 3)Check Balancen 4)Display all detailsn 5)Exitn" << endl;
cin >> c;

if(c == 1)
w.withdrawMoney();
else if (c == 2)
d.depo();
else if(c == 3)
b.check();
else if(c == 4)
b.display();
else if(c == 5)
exit(0);
else
cout <<"Wrong choice." << endl;
cout<<"Press any key to continue" << endl;
getch();
}
while(1);
getch();
return 0;
}

执行提款功能时，我得到以下输出：

你不能提取那么多。您的余额仅为 6.03937e-039

使用存款功能时，输出显示存款金额而不是实际余额。

您成功存入了1000。您的余额现在为 1000。

两个子类使用的唯一变量是bal，所以我决定像这样全局声明它。

#include<iostream>
#include<conio.h>
#include<stdlib.h>
float bal;

该程序没有任何缺陷。但是这样做违背了使用继承的全部目的。

我很困惑。为什么会这样？