const function what for?

[phuongtam]

What's the purpose of adding const to a function?


const_iterator begin() const { return data; }


I'm returning data so what's the difference if I ommit const? I don't understand what's used for..

[Duthomhas]

The const keyword is used to mark a thing as “promise not to change its value”.

So, const int x = -7; basically says you are expected to never change the value of x, and the compiler will do its best to hold you to that.

But what about complex types?

struct point
{
  double x, y;
  double hypotenuse() { return sqrt( x*x + y*y ); }
};

So far, so good. But now you have:

const point p { 3, 4 };
cout << p.hypotenuse() << "\n";

The compiler will complain. Why? Because that hypotenuse function does not promise not to change p.

Well, it doesn't. So let’s fix the code to make that promise.

struct point
{
  double x, y;
  double hypotenuse() const { return sqrt( x*x + y*y ); }
};

Now we have a member function that promises not to change its object, and the compiler is happy to let you use it on any point, both const and non-const.

The final thing to remember about this subject is that a non-const object can be temporarily marked as const.

void print( const point& p ) // I promise not to change the point I'm referencing here..
{
  cout << p.hypotenuse() << "\n";
}

int main()
{
  point q { 3, 4 };  // q is not const
  print( q );  // but function "print" will treat it as const
}

Hope this helps.