Help me understand this

[yasar11732]

Hi,

I have been reading this: https://www.cs.bu.edu/teaching/cpp/inheritance/intro/
and they have an example like this:

class Employee {
public:
  Employee(string theName, float thePayRate);

  string getName() const;
  float getPayRate() const;

  float pay(float hoursWorked) const;

protected:
  string name;
  float payRate;
};

What is the function/purpose of "const" at the end of method definitions like above?

[clanmjc]

The purpose of const on member functions is to identify which member functions may be invoked on const objects. They make the interface of a class easier to understand. It is important to know which funtions may modify an object and which may not. They make it possible to work with const objects. That is a critical aspect of writing efficient code because one of the fundamental ways to improve a program's performance is to pass objects by reference-to-const. This is only viable if there are const member function with which to manipulate the resulting const-qualified objects.

If in that example there was a function:

float pay(float hoursWorked);

//The signature for these two methods are different, they are 2 separate functions.
float pay(float hoursWorked) const;

The non const function could change the objects state (so could the const one with const casting but that is another can of worms)

Lets say you had a function that does something with an employee object.

void DoSomethingWithEmployee(const Employee &theEmployee);

If getName()const, getPayRate()const, pay(...)const were not const, you would not be able to do anything with that object in that function.

hope this helps.

[MrHutch]

It usually denotes that the function is read-only.

This means that it won't modify anything.

In your example, the first two functions are access functions, so it makes sense for them to be const. Not sure about the pay() function, though. Hard to tell without the declaration, really.

[clanmjc]

Not sure about the pay() function, though. Hard to tell without the declaration, really.
ico

The pay function, while making modifications to something, still maintains the logical constness of the object. It looks like it takes in hoursWorked, probably uses the payRate and calculates the actual "paid" amount and returns it. Making no changes to the object, thus maintaining it's contract as const.

[yasar11732]

@clanmjc:
Thanks a lot for your good explanations.

[ThangDo]

Declaring a member function with the const keyword specifies that the function is a "read-only" function that does not modify the object for which it is called.

This is an example from http://msdn.microsoft.com/en-us/library/6ke686zh(v=vs.80).aspx
It might help

// constant_member_function.cpp
class Date
{
public:
   Date( int mn, int dy, int yr );
   int getMonth() const;     // A read-only function
   void setMonth( int mn );   // A write function; can't be const
private:
   int month;
};

int Date::getMonth() const
{
   return month;        // Doesn't modify anything
}
void Date::setMonth( int mn )
{
   month = mn;          // Modifies data member
}
int main()
{
   Date MyDate( 7, 4, 1998 );
   const Date BirthDate( 1, 18, 1953 );
   MyDate.setMonth( 4 );    // Okay
   BirthDate.getMonth();    // Okay
   BirthDate.setMonth( 4 ); // C2662 Error
}

[MrHutch]

@clanmjc Ah, I see. Excellent, thanks for clarifying.