C++ question

hertz

When you have a class, and you have a member function whats the difference in having const before the function name compared to after the function name

EG

const void time::gettime (); const; // what do each of the two const's do in this case??

thank you.

hertz.

paddymee

const void time::gettime (); const; // what do each of the two const's do in this case??

Are you sure this even compiles?

Is this meant to be in the header or cpp file? It doesn't look right for either place.

hertz

Sorry, my fault, Il re phrase it..

const void time::settime(); // whats the difference in having const preceding this function

compared to this:
void time::settime() const;

Deitel & Deitel have this. I think the first one is to make the function constant to the client. The second Im not sure about, but it was in the book.

Thanks.
Hertz.

paddymee

const void time::settime(); // whats the difference in having const preceding this function

compared to this:
void time::settime() const;

Deitel & Deitel have this. I think the first one is to make the function constant to the client. The second Im not sure about, but it was in the book.

Ok, I can't tell you the difference because I dont know what the first is, but I reckon I know what the second is.

time time::gettime() const; means that the gettime member function will not modify the state of the time object. It's a way to get the state of an object and assuring the client that the state won't change because of this request.

Your example doesn't really make sense if I'm correct! It should return something and shouldn't set anything.

I suspect that the first is another way of typing the second, but I dont know.

I could be wrong about all this, so your best bet is to try it out and see what happens.


paddy

Lemming

Forgive me if I'm wrong ... (i've not been doing C++ in quite a few months, so I'm a little cold regarding its finer details), but isn't 'const' used to ensure that only one variable/object/etc can be in existence??

Eg. int blah const ;

and that a duplicat can't be created??

Although I've a feeling that paddymee is right in his last post about not changing states.

Typedef

Lemming I could be wrong , but I don't think so?

If you were intent on modifying the value of the of the time object couldn't you just do a hack like so?

#include <iostream.h>
#include <stdlib.h>

class time{
	public:
	int a;
	time()
	{
        a=100;
	};
	const time timex(time*x);
	void isonmyside(void) const;
	};

const time time::timex(time*x)
{
 x->a=99;
 cout<<"The value of the variable a is "<<(*this).a<<endl<<" "<<x->a<<endl;
return (*this);
};

void time::isonmyside(void) const
{
 time *n;
 //(*this).a=100 not allowed right?
 n=(time*)&(*this);
 n->a=899;
 cout<<"The value of the variable a is "<<(*this).a<<endl;

return;
};

int main(int argc,char**argv)
{
 time t,w;
 int a,b;
 cout<<"Objects instanciated printing values now "<<t.a<<" "<<w.a<<endl;
 t.timex(&w);
 t.isonmyside();
 cout<<"Main The value of time t.a is "<<t.a<<endl;
 cout<<"Main The value of time w.a is "<<w.a<<endl;
 system("pause");
return 0;
}; 

paddymee

Forgive me if I'm wrong ... (i've not been doing C++ in quite a few months, so I'm a little cold regarding its finer details), but isn't 'const' used to ensure that only one variable/object/etc can be in existence??

No, this isn't true. The const has many different meanings depending on its usage. I think you are thinking about assigning a value to a variable that cant be changed. Like the #define in ANSI C.

The const can be very confusing. The example, const void time::settime();. I don't know what it means.

voivoid time::isonmyside(void) const
{
time *n;
//(*this).a=100 not allowed right?
n=(time*)&(*this);
n->a=899;
cout<<"The value of the variable a is "<<(*this).a<<endl;

return;
};

Yes you can do this. But why would you? If its an accident, that shows why C casting is not safe. A static_cast would catch this.

If you are doing it on purpose, why bother making a "contract" by putting a const there and then going behind it's back?

Paddy.

Lemming

AH ... both myself and paddymee were right in our own way.

I've just checked my little reference book (a bargain at £15 for all the handy stuff in it).

"The C/C++ Programmer's Reference"
Herbert Schildt
Osborne publishing

In it, it states that const is used to tell your compiler that a variable cannot be changed by the program. Therefore that implies that only one instance of that variable can exist during the lifecycle of the program, AND the value can't be altered

paddymee

In it, it states that const is used to tell your compiler that a variable cannot be changed by the program. Therefore that implies that only one instance of that variable can exist during the lifecycle of the program, AND the value can't be altered

const has many different meaning based on it's context. The most basic form is that the variable can't be modified. It gets more complicated when pointers are involved. And functions etc..

However I don't see how you imply that there can only be one instance in the whole lifecycle of the program because it's const. That sounds like a STATIC to me.

Maybe you are confused there?

There is always one instance of the variable per instance of the class. Again unless it's a static, which is shared between instances.

Paddy

Lemming

Originally posted by paddymee

However I don't see how you imply that there can only be one instance in the whole lifecycle of the program because it's const. That sounds like a STATIC to me.

Maybe you are confused there?

Well .. I'm pretty sure I recall using it in a progam I did back in college to ensure that the EXACT same variable was being called even though it was being re-created (or something .. my memory get s alittle hazy, but I recall having to use it cause something was tryign to create an object was trying to create a second instance of that variable and .. OH .. now I think I recall more. I had an array of objects. But I needed a counter for something inside each of them that referenced a specific thingymabob .. I had to use const to ensure that the variable wasn't being recreated by each object)