Access an object from another file.

[cbarmpar]

Hi all,

I am new to Linux and c++ programming.

What I am trying to do is to declare an object in a .cpp and be able to access it from main.At the moment my code creates a new object in the ena.cpp and cannot use the one from the alpha.cpp. How can i access the object from alpha.cpp (bita.dio=6) from the main (ena.cpp).

Many thanks in advance.

ena.cpp

#include <iostream>
#include "alpha.h"
#include "bita.h"  
using namespace std;

int main() 
{
  A ob(4);
  B bita;//<this creates a new project which is irrelevant of the one i need to pass from alpha.cpp
  			// i need to remove that
  cout << ob.get_a();
  cout << bita.dio;//<-- This currently outputs 2 (and not 6 as i want)
    //cout << par;
  return 0;
}

alpha.cpp

#include "alpha.h"
#include "bita.h"
A::A(int x)
{ 
	B bita; 
    a = bita.ena;
   bita.dio = 6;//<---------- i want this to be accesible by ena.cpp directly
}

int A::get_a()
{
  return a;
}

alpha.h

#include "bita.h"
#ifndef ALPHA_H
#define ALPHA_H

class A {
  int a;
public:
  A(int x);
  int get_a();
};

#endif /*ALPHA_H_*/

bita.h

#include "bita.h"
#ifndef ALPHA_H
#define ALPHA_H

class A {
  int a;
public:
  A(int x);
  int get_a();
};

#endif /*ALPHA_H_*/

bita.cpp

#include "bita.h"

B::B(){
	ena = 1;
	dio = 2;
	tria = 3;
}

[gerard4143]

I'm not sure what your trying to do here....is this all the code your working with?

[cbarmpar]

Yes friend,

that's all the code.

I don't know how i can have access to an object created outside the ena.cpp.

Is it through the use of extern keyword how can i use the extern keyword with an object?

Regards

[gerard4143]

Oh...

You can use extern keyword but the convention is to use the #include "header.h"

I think this is what you want, if you have three programs:

main.cpp
one.cpp
two.cpp

and two header files

one.h
two.h

where main.cpp has the main function and includes one.h and two.h

#include "one.h"
#include "two.h"

int main(int argc, char**argv)
{
return 0;
}

to compile these files....

g++ -c one.cpp
g++ -c two.cpp
g++ -c main.cpp

g++ two.o one.o main.o -o main

Hope this helps

[cbarmpar]

many thanks for the reply.

Can you please show me an example on hot to use the extern keyword for my object. I can use it for variables but nit sure at all on hot to use it for objects.

Kindest regards

[gerard4143]

to use extern for an object you defined you would have to describe it to the compiler so that it could allocate the memory need...to do that you could typedef it but then you'd have to cast it to your object so you can use the functionality but if you can cast it to the object you defined then you wouldn't need the extern keyword....do you see the cycle here....

Why are you using extern?

[gerard4143]

I got this to work...but why would you duplicate the code with extern...why not just create a proper header file and include it....because the result is the same and a header file is easier to maintain

Note: you can remove the extern "C++" and this compiles fine

#include <iostream>

extern "C++" class a
{
public:
	a(int size);
	~a();
	int getx() const;
	void setx(int val);
private:
	int x;
};

int main(int argc, char**argv)
{
	a mya(12);
	std::cout<<mya.getx()<<"\n";
	return 0;
}

[gerard4143]

object with class definition

class a
{
public:
	a(int size);
	~a();
	int getx() const;
	void setx(int val);
private:
	int x;
};

a::a(int size)
:x(size)
{}

a::~a()
{}

int a::getx() const
{
	return x;
}

void a::setx(int val) 
{
	x = val;
}

[cbarmpar]

Many thanks for your time.

I think i now know what you mean. What i actually wanted is to create an object in a header file lets say header1.h implement it in header1.cpp. Make some calculations and alterations in header1.cpp and then be able to access this object from the main.cpp directly.

Behind all that is a project in qt. When i have a form i need to make an object which belongs to this form (and hence to this class) accesible by the main.

Many thnaks for your time once again

[gerard4143]

Make some calculations and alterations in header1.cpp and then be able to access this object from the main.cpp directly.

are you talking about self modifying code? i.e. being able to change header1.cpp while the program is running and see the changes in the running executable?
That is to say without exiting the program and re-compiling the project.
If this is what you want then good luck...you'll have to defeat the OS and few other things to manage that....

Or are you talking about two separate executables communicating via some interprocess communication IPC?