overloading operator <<

[jc82]

Hello all, I'm a C++ rookie learning about operator overloading so bare with me here. I am trying to find some help on how to use the overload operator << in ways other then to stream something to cout. An example would be having 2 account balances and trying to transfer the balance of one to the other.

So in main you would see the line something like this:

account2 << account1;

I guess I have 2 questions on this. The first, is how would I go about declaring the << operator in a header file?

friend ostream& operator<<(ostream& , const Account&); would work if I was streaming something to cout. But, I'm confused as to how you go about this when cout isnt involved. And, I'm also looking for ways to write the member function to get something like account2 << account1 to work. Thanks.

[vlad from moscow]

For example you can define it the following way

Account & Account::operator << ( const Account &rhs )
{
   return ( *this = rhs );
}

Or if you want that the operator would be a non-member function then

Account & operator << ( Account &lhs, const Account &rhs )
{
   return ( lhs = rhs );
}

Instead of assignment you can write any code and return the reference to the left operand.

[MrHutch]

I would make an account class and overload the operator in that class. Presumably, when you "transfer" a balance, you also want to add to the balance in the account transferring to and deplete the balance of the other account. This behavior should be represented in the overload.

Here's a quick example:

#include <iostream>
using namespace std;

class Account
{
private:
   double balance;
public:
   Account():balance(0.00){};
   void SetBalance(double);
   double GetBalance();
   void operator << (Account &);
};

void Account::SetBalance(double _balance)
{
   balance = _balance;
}

double Account::GetBalance()
{
   return balance;
}

void Account::operator << (Account &other)
{
   balance += other.balance;
   other.SetBalance(0.00);
}

int main()
{
   Account A, B;
   A.SetBalance(1200.00);
   cout << "Balance of A: " << A.GetBalance() << endl;

   B.SetBalance(500.00);
   cout << "Balance of B: " << B.GetBalance() << endl;

   A << B;
   cout << "New balance of A: " << A.GetBalance() << endl;
   cout << "New balance of B: " << B.GetBalance() << endl;

   return 0;
}

[vlad from moscow]

@iHutch105,

I am sorry but your code is very bad!

[jc82]

Thanks for the replies. I got it figured out now.

[MrHutch]

[sarcasm]Wow, Vlad, what in informative and insightful piece of criticism.[/sarcasm]

To what are you referring? The fact that one account has the authority to wipe the balance of the other due to the lack of a const keyword? As mentioned in the original post, it was a quick example and any specifics really should be addressed by the OP when implementing it specifically.

But if you want to be specific, I could have quite have easily pointed out that you're code can also be considered 'very bad'. I say this because if you work in the bank of Vlad and want to transfer a balance, like the OP said, then you may be somewhat dismayed to find out that all of your account details, including account numbers and sort codes, have been overwritten by the bank's balance transfer algorithm.

But it would be pretty pedantic of me to point that out.

If you're going to criticise then please do, I encourage it. I genuinely think that learning from the advice and criticism of other is the best way to learn and this will be evident where other people have commented on my code in other posts. However, if you're going to leave snarky, one-liner comments than save a bit of time for both of us and don't bother.

[vlad from moscow]

iHutch105,
I am sorry that I did not pointed out why your code is bad.
Firstly it is a bad idea to assign names to variables that starts from underluing symbol as _balance, because such names are reserved for internal names of the compiler inplementation and system API.

Secondly the function

double Account::GetBalance()
{
   return balance;
}

should be decl;ared as const that is as

double Account::GetBalance() const
{
   return balance;
}

Thirdly, it is a bad idea to have return type void for the operator <<. So instead of

void Account::operator << (Account &other)
{
   balance += other.balance;
   other.SetBalance(0.00);
}

iy is obvious would be better to write

Account & Account::operator << (Account &other)
{
   balance += other.balance;
   other.SetBalance(0.00);

   return ( *this );
}

Also I think that tha assignment operator and the << operator shall be considered together because they have similar semantic.