What are these two lines of C++ code doing?

action_owl · 12-13-2009, 04:25 PM

I just started a new C++ class and we had an in-class exercise where the teacher handed out parts of the solution to the problem before we were done.
So I never fully understood what was going on in two parts, and I really would like to.

Code:

class SavingsAccount
{
public:
   SavingsAccount( double b )
   {
      savingsBalance = ( b >= 0.0 ? b : 0.0 );
   }

   SavingsAccount &calculateMonthlyInterest();
   static void modifyInterestRate( double );
   void printBalance() const;

private:
  double savingsBalance;
  static double annualInterstRate;

};

The lines in question are:

Code:

SavingsAccount &calculateMonthlyInterest();
//The class... then the address of calculateMonthlyInterest()?

Code:

void printBalance() const;
//what is the const doing after printBalance()?

Any explanations are very appreciated, in the hand out nearly every line of the code is commented except the ones I didn't understand.
This is not homework

johnsfine · 12-13-2009, 04:44 PM

The whole section of code you quoted is the definition of a class.

The lines you asked about are declarations of functions that are members of the class.

Code:

SavingsAccount &calculateMonthlyInterest();

The & means the function returns a reference to a SavingsAccount object, rather than returning a SavingsAccount object by value.

Code:

void printBalance() const;

First you should understand that a function that is a non static member of a class takes a pointer to an object of that class as an implied extra parameter. So printBalance() appears to take zero parameters, but it really takes one parameter.

If you declared just

Code:

void printBalance();

then the implied extra parameter is
SavingsAccount* this;

The const you asked about in

Code:

void printBalance() const;

causes that implied extra parameter to be declared as
SavingsAccount const* this;

Quote:

Originally Posted by action_owl

This is not homework

I don't care, because it looks like reasonable question for you to ask here even if it were part of a homework assignment. If some detail within an assignment confuses you, ask about it. That is not the same as asking people to do the assignment for you.

tuxdev · 12-13-2009, 04:48 PM

Code:

SavingsAccount &calculateMonthlyInterest();

This is a function returning a reference.. the last line is very likely "return *this;", similar to the assignment operator, the iostreams stream insertions, and the Named Parameter idiom.

Code:

void printBalance() const;

The "const" marks that this member function should not modify the context object, and the compiler enforces this assertion. Const-correctness is one of the most valuable features the C++ compiler offers.

action_owl · 12-13-2009, 05:09 PM

wow, thank you johnsfine and tuxdev,

You helped clear this up for me.

It appears that his C++ II class is going to be a lot harder than C ++ I, but that means I'm going to learn more as well.