Thread: dynamic #define

I have .h file that had a #define on top and 3-5 classes there and everything worked like a charm.
However, I realize that this what I had defined may need to change some times. So, could I use a global variable (bliax). I probably can, but I think I can use a static variable or something. What do you suggest?

I thought getting it from the BIG class that contains all the others, but I want to define an array (not really a vector), like

Code:

#define N 5

...
A array[N+1];

However A has some pure virtual functions and I remember correctly, I can not create an instance of an abstract class. Only play around with pointers or references.
I mean, if I try to do

Code:

class B : public A{
  B() { 
    array = new A[5];
     OR
    for(int i = 0 ; i < 5 ; ++i)
         vector = push_back((A()));
  }

I will receive compilation errors, so what do you suggest?

I also tried by removing the "=0", going from pure virtuals to just virtuals, but then I got

Code:

samaras@samaras-A15:~$ g++ -Wall px.cpp -o px
/tmp/ccDCzdcC.o: In function `A::A()':
px.cpp:(.text._ZN1AC2Ev[_ZN1AC5Ev]+0x8): undefined reference to `vtable for A'
/tmp/ccDCzdcC.o: In function `A::A(A const&)':
px.cpp:(.text._ZN1AC2ERKS_[_ZN1AC5ERKS_]+0x8): undefined reference to `vtable for A'
/tmp/ccDCzdcC.o: In function `A::~A()':
px.cpp:(.text._ZN1AD2Ev[_ZN1AD5Ev]+0x8): undefined reference to `vtable for A'
/tmp/ccDCzdcC.o:(.rodata._ZTI1B[typeinfo for B]+0x8): undefined reference to `typeinfo for A'
collect2: ld returned 1 exit status

As you can see, it can not find it to the vtable

EDIT: The title I gave can be a nominee for The Worst Title of cboard for 2013

For which of the two parts? For the abstract class creation error? If yes, take this similar simple main.cpp

Code:

#include <iostream>
#include <vector>

class A {
public:
        virtual void bla() = 0;
        friend std::ostream& operator << ( std::ostream& s, const A& p )  {
                s << p.a;
                return s;
        }
        int a;
};

class B : public A{
public:
        B(int a, int b) {
                array = new A[5];
        }
        virtual void bla() { std::cout << "I am bla\n" << std::endl; }

private:
        A* array;
};

int main()
{
        B b(2, 2);
        return 0;
}

Originally Posted by manasij7479

So, what is your question?
You already know you can not do that.

Do you want suggestions for alternative designs ?
If so, you have to specify what you are designing.

I think that I have to use something like a global variable, like the #define I had. But, because I had in mind that the #define can reduce my options, I have used it only inside the .h file.
The question is, what I should use, in order to replace the #define, but get in file (only) scope only?

Moreover, I wanted to be sure that I remember correctly, I can not do that.

EDIT: Almost forgot, I want to set that "global in file scoped" variable dynamically, in the constructor for example.

1)Good point.

2)I do not want. My initial purpose is to do in the private part of the class

Code:

#define N 5 // If I use here const, as you pointed, it will become a global variable, right?

AbstractClassType array[N+1];

That's what I have to do! But, I see that N has to be taken from input.

3)You mean initiazling the array from the in. list? How?

@India guy, this solution requires me to change a lot of code now and I think using a static (?) variable in the .h is going to be ok.

You mean like this?

Code:

#include <iostream>
#include <vector>

class A {
public:
        virtual void bla() = 0;
        friend std::ostream& operator << ( std::ostream& s, const A& p )  {
                s << p.a;
                return s;
        }
        int a;
};

class B : public A{
public:
        B() : array(new A[5]) {}
        virtual void bla() { std::cout << "I am bla\n" << std::endl; }

private:
        A *array;
};

int main()
{
        B b();
        return 0;
}

Same error as my approach

Code:

samaras@samaras-A15:~$ g++ -Wall px.cpp -o px
px.cpp: In constructor ‘B::B()’:
px.cpp:16:28: error: cannot allocate an object of abstract type ‘A’
px.cpp:4:7: note:   because the following virtual functions are pure within ‘A’:
px.cpp:6:22: note: 	virtual void A::bla()

a and b are left-overs from another example. I discarded them.

>Also are you sure you want to have an array of A in your B class?
>If so why do you have the inheritance?
I guess so, since I need an array, which will be of type of a base (abstract) class. Then there are two classes that inherit from this base. I want to cast depending on what I have in the .cpp. I do not feel like changing that now.

>What do you mean by "N has to be taken from input"?
I mean that N is not a constant as I thought in the start. Now, N is provided by the input. by the user.

Ok guys, thanks for the tips.

But, I want to test if my logic on the problem I am working on works. So, as a temporary solution, instead of modifying the #define by hand, can I do something else? Like having a global/static variable or something and initializing it in the constructor of one class (the BIG one) that is contained in this file?

Hmm, maybe you are right, but I do not know how to change it. The code is big, so I do not think that it would be appropriate to post it here. Anyway... I will think.

Originally Posted by std10093

The code is big, so I do not think that it would be appropriate to post it here. Anyway... I will think.

Then try to make a small, minimal example without the nitty, gritty details?
I also react on the fact that you want some array of size N + k of some pointers to an abstract class, but you later specify that N will change depending on input. Then what is the point of having N as a variable? You should probably pass size to these functions/classes that needs them directly and possibly notify them if the size of their internal array ("N") needs to change.
I also get somewhat suspicious when you say you want to cast pointers. Why?