declaring constants in classes

Posted on 1998-12-16
Medium Priority
Last Modified: 2010-04-02

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 ?
Question by:meessen
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LVL 22

Accepted Solution

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.
LVL 22

Expert Comment

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;

Expert Comment

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

class foo
 val1 = 5 ;
} ;


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

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Expert Comment

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.
       const int val = 5;

LVL 22

Expert Comment

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.

Author Comment

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 !
LVL 22

Expert Comment

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...

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