E2353 Class '.. ' is abstract because of...

[jewel87]

Hello,
I am trying to make an abstract base class and 3 derived classes. If i include the derived classes function declarations in the base class and make them virtual and =0, once i add more than one function, i get these errors:

E2352 Cannot create instance of abstract class 'CurrentAccount'
E2353 Class 'CurrentAccount' is abstract because of 'Account::create(double,AnsiString,int,double,Ansi String,double) = 0'

Here is the code. Base class:

 
class Account{
        protected:
                  String BankCode;
                  int AccountNo;
                  double Balance;
                  String BankName;
                  double Rate;
 
        public:
 
                virtual void create(String aBankCode,
                            int aAccountNo,
                            double aBalance,
                            String aBankName,
                            double aRate,
                            double anOverdraftLimit,
                            String aConditions)=0;   //implemented in CurrentAccount
 
               virtual void create(double aWithdrawLimit,
                                String aBankCode,
                                int aAccountNo,
                                double aBalance,
                                String aBankName,
                                double aRate)=0; //implemented in SavingAccount
 
                virtual void display(TLabel *BankCodeLabel,
                             TLabel *AccountNoLabel,
                             TLabel *BalanceLabel,
                             TLabel *BankNameLabel,
                             TLabel *RateLabel,
                             TLabel *OverdraftLimitLabel,
                             TLabel *ConditionsLabel)=0;
 
                virtual int CheckBalance()=0;
 
                void deposit(double Amount);
                void withdraw(double Amount);
 
                Account();
                ~Account();
 
 
};
 
 

Derived classes:

 
#include "Account.h"
 
class CurrentAccount: public Account{
 
        private:
                double OverdraftLimit;
                String Conditions;
 
        public:
                void create(String aBankCode,
                            int aAccountNo,
                            double aBalance,
                            String aBankName,
                            double aRate,
                            double anOverdraftLimit,
                            String aConditions);
 
                void display(TLabel *BankCodeLabel,
                             TLabel *AccountNoLabel,
                             TLabel *BalanceLabel,
                             TLabel *BankNameLabel,
                             TLabel *RateLabel,
                             TLabel *OverdraftLimitLabel,
                             TLabel *ConditionsLabel);
 
                double CheckInterest(double EnteredBalance, double theRate);
                int CheckBalance();
 
                CurrentAccount(String aBankCode,
                                int aAccountNo,
                                double aBalance,
                                double aRate,
                                double anOverdraftLimit);
                CurrentAccount();
                ~CurrentAccount();
 
};
 
 

 
#include "Account.h"
 
class SavingAccount : public Account{
 
                private:
                       double WithdrawLimit;
 
                public:
                       void create(double aWithdrawLimit,
                                String aBankCode,
                                int aAccountNo,
                                double aBalance,
                                String aBankName,
                                double aRate);
 
                       void display();
                       void withdraw();
                       SavingAccount();
                       ~SavingAccount();
 
};
 
 

Someone please help!!!!

[jewel87]

People, is there anyone to help me? Please, this is very urgent! How to make derived class functions overload the base class function?? They all have different parameter sets, but i can't make it work!

[chroot]

Hi

As I see it, you have two functions void create(...) = 0;

You implement one of them in CurrentAccount and one in SavingAccount, but you would have to implement both functions in both classes to stop them from being abstract.

A class is abstract until all functions virtual ... =0; of all base classes are implemented.

Why do you need both functions create to be virtual .. = 0; ?

[jewel87]

Hi

As I see it, you have two functions void create(...) = 0;

You implement one of them in CurrentAccount and one in SavingAccount, but you would have to implement both functions in both classes to stop them from being abstract.

A class is abstract until all functions virtual ... =0; of all base classes are implemented.

Why do you need both functions create to be virtual .. = 0; ?

What the task was saying, i have to use this code:

 
Account ** theAccounts; // declare in the Employee class (other class connected with Account base class)
 
// In the Employee constructor, allocate memory for
//each element of theAccounts array:
theAccounts = new Account * [3];
 
// Create 3 different accounts for the employee
//object:
theAccounts[0] = new CurrentAccount();
theAccounts[1] = new ISAAccount();
theAccounts[2] = new SavingAccount();
 
// De-allocate memory for theAccounts dynamic
//array in the Employee destructor
delete theAccounts[0];
delete theAccounts[1];
delete theAccounts[2];
delete theAccounts;
 
 

And then access each derived class object from Employee class like this, which should use each derived class own function:

 
theAccounts[0]->create(EnterBankCode->Text,
                                StrToInt(EnterAccountNo->Text),
                                StrToFloat(EnterBalance->Text),
                                EnterBankName->Text,
                                StrToFloat(EnterRate->Text),
                                StrToFloat(EnterOverdraftLimit->Text),
                                EnterConditions->Text); 
 

For each derived class object i have different sets of arguments in the function,
but if i don't declare them in the base class, i can't access them here. What should i do????? I'm confused, and the teacher is not helping... i need to submit this by tomorrow morning...;(((

[chroot]

Well, the important thing about using polymorphism (which the exercise is about, I suppose), is that the base class ( Account ) can only have functions and members that every class that inherits from it has too.

So, if you want to be able to call Accounts[0]->create( ... ), you must have one function create which takes the same arguments for each class. Only then it is possible to call the function for any Account (CurrentAccount or SavingAccount).

I would try to write a function create( ... ), taking all arguments common to both CurrentAccount and SavingAccount. This function can be overloaded by both functions.

You'll have to do the same for display and CheckBalance, i.e. implement them in each class with the same arguments.

I hope things got less confusing.

[jewel87]

Well, the important thing about using polymorphism (which the exercise is about, I suppose), is that the base class ( Account ) can only have functions and members that every class that inherits from it has too.

So, if you want to be able to call Accounts[0]->create( ... ), you must have one function create which takes the same arguments for each class. Only then it is possible to call the function for any Account (CurrentAccount or SavingAccount).

I would try to write a function create( ... ), taking all arguments common to both CurrentAccount and SavingAccount. This function can be overloaded by both functions.

You'll have to do the same for display and CheckBalance, i.e. implement them in each class with the same arguments.

I hope things got less confusing.

Yeah, thanks, you made it more clear, but what should i do with the data specific for each derived class? i need to pass to each new object additional arguments depending on the class...

[chroot]

Well the easiest way would be to just add more arguments to your function:

create( commonArgument1, commonArgument2, ..., CurrentAccountArgument1, CurrentAccountArgument2, SavingAccountArgument1);
 

Than, in the overloaded functions, you just ignore the arguments that concern other classes (CurrentAccount::create will ignore SavingAccountArgument1), and give any data when calling this function.

However, this is a very ugly way to do it, as a potential user of the class would get really confused about which arguments are going to be ignored.

In fact, it would be important to know from where you call those create() functions, how you know which kind of Account to create, etc.

You could also make specific functions for each inherited class, which set the specific arguments. It depends on how you're going to use the class.