C++: Multiple definition error when including parent class? - c++

Question

I am writing a simple banking program with derived classes and I am running into a Multiple definition of <method name> error when including parent class.
Keep in mind that I just started coding in C++ yesterday, and moving over from Java/PHP, handling headers/definitions is a bit confusing for me. Please correct anything you see wrong!
Here is a sample of my files/code:
Files
Account.h
Account.cpp (Super)
ChequingAccount.cpp (Child)
SavingsAccount.cpp (Child)
The error is reproduce-able when including the parent class (Account.cpp) into any file. I have reduced my code by a lot, but it should give you an idea of how I am handling inheritance.
To clarify, when I #include the child classes to any file (ChequingAccount.cpp) works fine, and inherited functions work as expected. However, when I #include the parent class (Account.cpp) breaks the compiler with the Multiple definition of <method name> error for all methods.
Again, I am not sure if this is the proper way to do it, but this is what I understand from tutorials I have found.
Code
Account.h
#ifndef ACCOUNT_H
#define ACCOUNT_H
class Account
{
protected:
double m_balance;
public:
Account(double balance); // Constructor.
virtual ~Account(); // Destructor.
// Accessor Methods.
double getBalance() const;
// Mutator Methods.
virtual void withdrawFunds(double amount);
void depositFunds(double amount);
};
#endif
Account.cpp (Superclass)
#include "Account.h"
Account::Account(double balance = 0)
{
m_balance = balance;
}
Account::~Account()
{
// TODO: Delete this data structure...
}
double Account::getBalance() const
{
return m_balance;
}
void Account::withdrawFunds(double amount)
{
m_balance -= amount;
}
void Account::depositFunds(double amount)
{
m_balance += amount;
}
ChequingAccount.cpp (Child)
#include "Account.h"
class ChequingAccount: public Account
{
public:
ChequingAccount(int id, int userId, double balance) : Account(id, balance){};
void withdrawFunds(double amount)
{
// Override parent method.
}
};
Any help would be greatly appreciated! Thank you!

Answer 1

When you #include "some file.cpp", you are directing the compiler to copy the contents of that cpp file to that point in the program. This will create two compiled versions of your "some file" which will lead to "Multiple Definitions."

Answer 2

First of all, "identical" functions declared in base class and sub-class will not cause multiple definition error.
Here is just one example and explanation aiming to help you understand my point:
main.cpp
class father{
void fun();
}
void father::fun(){}
class son : public father{
void fun();
}
void son::fun(){}
int main()
{
return 0;
}
You compile it, and definitely no multiple definition error. In Java, we define functions within classes. In C++, only inline functions are defined within classes and others are declarations, which can be declared whatever times you like. See the definition syntax : father::fun son::fun. These actually define two different functions, one is in father and the other is in son. So there are no multiple definition error. Once more, in class declarations you can only define inline functions and can only declare non-inline functions. So there are on multiple definition errors at all. This just aims to help you understand it in a grammar way.
BTW, if compiler failed to inline, there would be no multiple definition error too because even though you define inline functions in header files and include the files anywhere, inline functions are of internal linkage.
I compiled your codes and got different errors regarding your defined constructors. Anyway, including cpp files using "include" means you don't get the point of implementation file and interface file.

Answer 3

You need to declare the child's method as a member of the child class explicitly in the child's .cpp file as well. See below:
ChequingAccount.cpp (Child)
#include "Account.h"
class ChequingAccount: public Account
{
public:
ChequingAccount(int id, int userId, double balance) : Account(id, balance){};
void ChequingAccount::withdrawFunds(double amount)
{
// Override parent method.
}
};

Answer 4

You already have void Account::withdrawFunds(double) defined when you define void ChequingAccount::withdrawFunds(double).
Try virtual void ChequingAccount::withdrawFunds(double)
The virtual keyword is similar to overriding in Java.

Answer 5

In short, you should not include a cpp file.
The reason you get a multiple definition error actually has nothing to do with class inheritance.
Consider a simple example:
add.h:
#ifndef __ADD_H_
#define __ADD_H_
int add(int a);
#endif
add.cpp:
#include "add.h"
int add(int a) { return a + 1; }
main.cpp:
#include "add.h"
int main() { return add(-1); }
The way C++ dealing with #include directive is simply to copy-paste the included file and replace the #include line. So if we expand the above files manually, we will get something like:
add.cpp:
#ifndef __ADD_H_
#define __ADD_H_
int add(int a);
#endif
int add(int a) { return a + 1; }
main.cpp:
#ifndef __ADD_H_
#define __ADD_H_
int add(int a);
#endif
int main() { return add(-1); }
which will work just fine. (We are allowed to have multiple declarations of a function)
However, if you decide to include the .cpp file instead of the .h file like the code below:
main.cpp:
#include "add.cpp" // notice here
int main() { return add(-1); }
And if you expand it as we just did:
add.cpp:
#ifndef __ADD_H_
#define __ADD_H_
int add(int a);
#endif
int add(int a) { return a + 1; }
main.cpp:
#ifndef __ADD_H_
#define __ADD_H_
int add(int a);
#endif
int add(int a) { return a + 1; }
int main() { return add(-1); }
You will see the code includes multiple definitions of the function add, one of which is in add.cpp and the other one is in main.cpp. When linking these two files together (each .cpp file is compiled separately using a compiler and then linked together using a linker), the linker will be confused by two definitions of add and doesn't know which one to use, so it will start to complain.