CODEWITHSUNDEEP
c++privatescoping
paxilpaz
paxilpaz

Reputation: 431

C++ - Access private members of class from outside the class

I'd like to know if there's any way to access a private member of a class from outside the class. I'll explain my problem.

I have a .hpp file which contains the definition of the class along with its private members and public function (which are the only one I'd like to export). In the corrisponding .cpp I have to use some "support" function which need access to the private members of the class defined in the .hpp.

Here's part of my code:

--- .hpp ---

namespace vision {
class CameraAcquisition {

    /* MEMBERS */
    CvSize size;
    CvCapture *device;
    CvScalar hsv_min,hsv_min2,hsv_max,hsv_max2;

    /* METHODS */

public:
    CameraAcquisition();
    ~CameraAcquisition();
    int findBall();
};
}

--- .cpp ---

#include "CameraAcquisition.hpp"

using namespace vision;

IplImage *grabFrame() {
     // code here
}

IplImage *convertToHSV(IplImage *origin) {
// code here
}

IplImage *calculateThresholdedImage(IplImage *converted) {
// code here
}

What I need is for these three functions to access the members of the class CameraAcquisition. Is there any way to do it? Any suggestions will be appreciated. Thank you all

EDIT Sorry, I forgot an important piece of information here. In the source file, findBall() must call those methods. I defined those methods to make the code easier to read. I cannot declare those methods in the class definition because I don't want to export them. If I declare them in a "private" block everything works fine, but maybe it's not correct to that (I don't see the point in providing an header file with private methods.

Upvotes: 4

Views: 14258

Answers (5)

Santosh Kale
Santosh Kale

Reputation: 113

Compiler used is Visual Studio 2015
This method is not recommended but works.
Lets consider following class

class mydata
{
    private:
    int x;
    public:
    mydata(int no)
    {
        x=no;
    }
}

Only data members are stored in the class object. Now I can access the x using following function. As I know that class mydata has only one variable it must be the int x.

int getx(mydata *d)
{
    return ((int*)d)[0];

    /*
    How did this work?
    -> d is pointing to the mydata object. 
    as I typecasted it to int* it will be considered as int array.
    Of that array (which has lenght 1) access the first element.
    */
}

If there was another variable lets say y of type DATATYPE.
Now, to access y we have to calculate the offset of it from the base of object.

Usually data is stored in the same order in which you declare it.
Also, there is struct padding to be considered in case of heterogeneous data types in the class. I would like to suggest you to read struct padding in depth.

And the we can get y as 

myclass *ptr =new myclass();
int offset_y=//whatever offset of y in number of bytes from base of object of perticular class;
char *byte_ptr_y=((char*)ptr)[offset_y];
DATATYPE y=*((DATATYPE*)byte_ptr_y);

Upvotes: 0

slartibartfast
slartibartfast

Reputation: 4428

You could always declare them as friends by adding in the class definition:

friend IplImage * ::grabFrame();

Upvotes: 0

Carl Winder
Carl Winder

Reputation: 948

Use getters in your CameraAcquisition class

i.e.

CVSize getSize() { return size; }

CvCapture* getDevice { return device; }

etc...

Upvotes: 0

Attila
Attila

Reputation: 28762

You should not want to access private mebers of objects. Consider providing public getter/setter member functions that outside code can use to affect the private member variables

Alternatively, you can make your support function a member of the class, so it has automatic access to the private members.

If you really want to access private members, you can use the friend keyword to declare another class or function as friend of the current class

class A
{
  int a; // private
  friend class B;
  friend void f(A*);
};

Here both class B (i.e. its member functions) and free function f can access A::a

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258548

If those members are made private and you need access to them, you're doing something wrong, so my suggestion is that you don't.

Now let's get serious - there are some ways - DISCLAIMER: not all are safe or portable or guaranteed to work:

1) Preprocessor abuse

#define class struct
#define private public
#include "CameraAcquisition.h"
#undef class
#undef private

2) Fake class

class MyCameraAcquisition {
public: //<--- public in your class
    /* MEMBERS */
    CvSize size;
    CvCapture *device;
    CvScalar hsv_min,hsv_min2,hsv_max,hsv_max2;

    /* METHODS */

public:
    CameraAcquisition();
    ~CameraAcquisition();
    int findBall();
};

CameraAcquisition prvt;
MyCameraAcquisition publ = (MyCameraAcquisition&) prvt;

3) Template abuse posted by litb - http://bloglitb.blogspot.com/2010/07/access-to-private-members-thats-easy.html

This one is, actually, and surprisingly, standard-compliant.

Upvotes: 3

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