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Static data inheritance problem

 
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Sue D. Nymme



Joined: 26 Jul 2006
Posts: 13
Location: Philadelphia, PA, USA
PostPosted: Fri Jul 28, 2006 7:41 am    Post subject: Static data inheritance problem Reply with quote
Hello, I have a class hierarchy in which each class has some data that is constant for the class (but which varies from class to class). I have boiled it down to the following example program:

Code:

import std.stdio;


class Parent
{
    static char[] info = "Parent class";

    this()
    {
        writefln(`Constructing a ?s. info is "?s"`,
                 this.classinfo.name, this.info);
    }

    void print()
    {
        writefln(`I am a ?s. My info is "?s"`,
                 this.classinfo.name, this.info);
    }
}

class Child : Parent
{
    static char[] info = "Child class";
}

void main()
{
    Parent P = new Parent;
    Child  C = new Child;

    P.print();
    C.print();
}

The output I would like to see is:
Quote:

Constructing a Parent. info is "Parent class"
Constructing a Child. info is "Child class"
I am a Parent. My info is "Parent class"
I am a Child. My info is "Child class"

The actual output I am getting is:
Quote:

Constructing a Parent. info is "Parent class"
Constructing a Child. info is "Parent class"
I am a Parent. My info is "Parent class"
I am a Child. My info is "Parent class"

The base class methods seem to see only the base class static data.

If I copy the print() function from the base class into the child class, then C.print() does print
Quote:

I am a Child. My info is "Child class"


But I would like to avoid that -- the whole point of inheritance is not to duplicate code! Smile

Any help would be appreciated.
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pragma



Joined: 28 May 2004
Posts: 607
Location: Washington, DC
PostPosted: Fri Jul 28, 2006 7:53 am    Post subject: Reply with quote
Static members aren't placed in the class' v-table, so they're not inherited/overridden in the same way as members. In your code above, the reference to "this.info" is equivalent to "Parent.info".

Anyway, just wrap your reference to 'info' with a member instead, and all will be well. Constant strings are by definition stored statically, so we can do away with the 'static char[]' stuff and get right to it:

Code:

class Parent
{
    char[] info(){ return "Parent class"; }

    this()
    {
        writefln(`Constructing a ?s. info is "?s"`,
                 this.classinfo.name, this.info);
    }

    void print()
    {
        writefln(`I am a ?s. My info is "?s"`,
                 this.classinfo.name, this.info);
    }
}

class Child : Parent
{
    char[] info(){ return "Child class"; }
}

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Sue D. Nymme



Joined: 26 Jul 2006
Posts: 13
Location: Philadelphia, PA, USA
PostPosted: Fri Jul 28, 2006 9:46 am    Post subject: Reply with quote
Thanks pragma. Works like a charm.
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