?
Solved

declaring constants in classes

Posted on 1998-12-16
7
Medium Priority
?
324 Views
Last Modified: 2010-04-02
Hello,

I'm currently porting C++ program from unix to NT and discover the following problem.

With gcc one can declare and initialize a constant value in a class. Apparently VC++ does not allow this.

class foo {
   const int val = 5;
}

This makes VC++ complain.
How do we define class dependent constants with VC++ ?
What is the standard behaviour ?
0
Comment
Question by:meessen
7 Comments
 
LVL 22

Accepted Solution

by:
nietod earned 400 total points
ID: 1180285
the ability to declare and initalize a constant in a class was added to the standard, but is not supported by VC yet.
0
 
LVL 22

Expert Comment

by:nietod
ID: 1180286
in the meantime, you can use an enum like

class foo {
      enumt valType { val = 5};
   };

or you can use a static constant like

class foo {
      static const int val;
   }

const int foo::val = 5;
0
 
LVL 8

Expert Comment

by:Answers2000
ID: 1180287
You beat me to it nietod (just), oh well  a comment

class foo
{
enum
{
 val1 = 5 ;
} ;

//or

static const int val2 ;

}  ;

// then a .cpp
const int foor::val2 = 5 ;


I have read the gcc option is allowable but a recent addition to ANSI/ISO C++, so presumably a version of VC will eventually fix this.  Lot's of compilers currently barf on the const int declarations in a class


0
Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 

Expert Comment

by:midnitelite
ID: 1180288
since in VC you can't initial a constant, you can initial in the Constructor like in the following.  This is appropiate and should be used.



    class foo {
       // const int val = 5;
    }
    // put constant value in constructor.
    foo::foo()
    {
       const int val = 5;
    }


0
 
LVL 22

Expert Comment

by:nietod
ID: 1180289
that creates a constant that is local to the constructor procedure.  That is not what he wants.  You can declare a constant int member to the class and initialize it in the constructor.  however that is a waste of space as each object will store its own copy of this int.  That is why I suggested using a static int.
0
 
LVL 1

Author Comment

by:meessen
ID: 1180290
Ok nietod get the points though I used Answers2000 form:

enum { val = 5 };

I hope VC updates it's compiler really soon.
We have very programs here such constants are used everywhere !
0
 
LVL 22

Expert Comment

by:nietod
ID: 1180291
(I also proposed that solution, although I did not use an annonymous enum.)

>>I hope VC updates it's compiler really soon
they produce updates about every 12 to 18 months.  Last one was about 6 months ago...
0

Featured Post

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Unlike C#, C++ doesn't have native support for sealing classes (so they cannot be sub-classed). At the cost of a virtual base class pointer it is possible to implement a pseudo sealing mechanism The trick is to virtually inherit from a base class…
Many modern programming languages support the concept of a property -- a class member that combines characteristics of both a data member and a method.  These are sometimes called "smart fields" because you can add logic that is applied automaticall…
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…
The viewer will learn how to pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.
Suggested Courses

840 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question