Urgent! C++ How to declare/initialize a constant in a class?

Mr Virtual

1. The problem statement, all variables and given/known data
I have my computer science exam today (in 2 hours). I want to know how to declare and initialize a const member in a class.

3. The attempt at a solution

I tried this:

__________________________________________________ _______________
class stu
{
const int a=10; // Error cannot initialize a data member here
};
--------------------------------------------------------------------------

Then I tried this:
__________________________________________________ _______________
class stu
{
const int a;
};

int stu::a=10; //Error : Multiple declaration of stu::a
--------------------------------------------------------------------------

Then this:
__________________________________________________ _______________
class stu
{
const int a;

stu() //constructor
{
a=10; //Error: Cannot modify a const member
}

};

--------------------------------------------------------------------------

Please help!!!!

Thanks
Mr V

Mr Virtual

I am using Turbo C++ 3.01

Mr V

out of whack

IIRC, when you don't need external linkage you can do this:

class stu { static const int a=10; };

If you need external linkage (eg you're building a library that will be linked to other modules) you're safer with this form:

class stu { static const int a; };
const int stu::a=10;

...I think.

chroot

You're just missing the 'static' keyword.

- Warren

Mr Virtual

Thanks a lot! It works perfectly.
But I am a little confused as to why the keyword 'const' is preceded by 'static'. Can't I just create a const without making it a static? I am asking this because value of a const cannot be changed, then why do we need to make it a static?

Mr V

Mr Virtual

Yeah, I realised that. But can you explain why we need to make it a static here?
Is this to make the objects able to share the const.
Mr V

chroot

A static member belongs to the class; it is shared among all instances of the class. A non-static member belongs to a specific instance.

- Warren

Mr Virtual

One more question: if I declare a member function as 'static const' what does it mean? I know that since it is static, it is restricted to accept/access static data members only. But what effect does maknig it a 'const' have on it? Will it accept/access only const values?

Mr V

chroot

Static and const mean two different things. Static members belong to the class, rather than specific instances of the class. Static data isn't necessarily constant, though. A constant member is one that is created with a specific value, and whose value cannot ever be changed.

- Warren

Mr Virtual

Sure you are right. But does making a function constant restrict it to access only const members?

Mr V

chroot

No. If a function is constant, then it returns something (perhaps an object) which cannot be changed by the caller. It doesn't matter what data the function uses to create its return value.

- Warren

Mr Virtual

Oh!Oh! How did I forget that! I forgot that the datatype preceding function name specifies its return type. So, if I declare a function const, it will return a const value.

Thanks a lot to you, and 'out of whack' for all your help. I hope I would fare well in my exam. Bye then!

Mr V

chroot

Good luck!

- Warren

Mr Virtual

Thanks!
I performed well. Only forgot that 'setw' is in 'iomanip.h' and that puts() automatically includes a newline character at the end of a string.

Mr Virtual

I have my chemistry paper next, and I have a terrible confusion in Henry's law of solubility. I am creating the thread in the homework section (chemistry). Please help me there...

Mr V

chroot

Mr Virtual,

Those things don't matter anyway. No one uses puts(). I'm almost amazed teachers test on such useless things!

- Warren