change a parameter from a object from base class !!!

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1.00/5 (2 votes)

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Hi I have a problem
I have a base class :

C++

class Base
{
public:
    myClass* a;
    virtual void Update(){}

};

and another class :

C++

class NewClass:public Base
{
public:
    NewClass();
    virtual void Update()
    {
        a->val=200;
    }
}

now I do this :

C++

myClass* b;
Base me;
me=NewClass();
me.a=b;
me.Update();

but now b dont change. I want change b value. how can I do this like this ?

Posted 30-Dec-12 8:45am

mohammadali1375

Updated 30-Dec-12 22:37pm

v5

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H.Brydon · Answer 1 · 2013-01-01T06:23:00

I haven't tried it but I wouldn't expect your code to compile.

You are describing 3 classes here: Base, NewClass and myClass. Base includes a pointer to myClass, which in your code as provided, is not defined (error). If you somehow overlook this and define it somehow, in the lower section of code:

1    myClass* b;
2    Base me;
3    me=NewClass();
4    me.a=b;
5    me.Update();

Lines 2 and 3 could be combined as

2/3    Base me=NewClass();

which represents use of a copy constructor, building a new 'Base' object, copying elements from a new 'NewClass' temporary object. At this point, me.a is undefined (usually trash from the stack with most compilers). Line 4 should assign me.a the value of b (on the stack) and line 5 will change b->val.

Assumptions:
(1) We are talking C++ here. Java for example does something different.
(2) This code segment is external to the classes in question.
(3) myClass contains something named 'val' (which is well formed POD and not a property or something else funky).

You didn't provide the code for class myClass, which could change all of the above.

Mattias Högström · Answer 2 · 2012-12-30T09:26:00

Solution 1

I am sorry, but you have confused the usage of pointers

C++

a=200; // is wrong

you probably want to do

C++

*a = 200;

Posted 30-Dec-12 9:26am

Mattias Högström

Updated 30-Dec-12 11:33am

v3

Comments

mohammadali1375 31-Dec-12 4:37am

Oh I'm sorry. I try make an example from my problem quickly . Now I edit my question;

Mattias Högström 31-Dec-12 10:25am

The code didn't become clearer.

Theoretically, the assignment looks ok now,
but your code contains a couple of errors and omissions.
I don't think it compiles, so I am not sure what the error is.

1.
Allocate myClass object or use an object.
myClass* b = new myClass();
myClass b;

2.
When using subclasses and virtual methods, a pointer must be used.
Base me;
me=NewClass();
=== Change to ===>
Base *me = newClass();

3.
Exposing a public variable is bad practise, and of pointer type, is worse.
If you really need to pass around pointers, use the constructor and/or get set methods.
The reference type (myClass& par) is even better than pointers because it can't be null.
A "smart pointer" is even better to use than raw pointers.
http://en.wikipedia.org/wiki/Smart_pointer