how to use member variables in another class

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Hi all,
I have some trouble with my program.I wrote two classes

class LoadImage
{
 public:
     ......
     int width;
     int height;
     Byte *imgData；
};

In the class above I load an image,and get the information about width, height of the image,I want to process the image in the following class;

class ProcessImage
{
 ....
};
void ProcessImage::onProcess()
{
  LoadImage img;
  int lwidth=img.width; // this is the wrong way I used!!
  ...
}

in the class ProcessImage, I will use the member variables in class LoadImage frequently,but after I load the image, the member variables in class LoadImage are all destroyed, how can I use these variables when I process the image?

Posted 5-May-13 4:10am

Angela2012

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Comments

Richard MacCutchan 5-May-13 11:38am

You need to actually load an image into your LoadImage object, and initialise the variables with the relevant information. As it stands your code does nothing.

Maciej Los 5-May-13 12:29pm

Please, see this: Image Class (System.Drawing)[^].

Ian A Davidson 5-May-13 20:37pm

This is C++, not C#.

Maciej Los 6-May-13 1:33am

My little mistake because of C++ tag. Simply change C++ tab to C# tab ;)

2 solutions

Solution 1

You declare LoadImage but you never create the associated object. You need to create the object so that you can access the member data, like:

LoadImage img = new LoadImage();
int lwidth = img.Width;

Otherwise you are attempting to access protected memory or it could even be some random value because its not initialized.

Posted 5-May-13 13:44pm

Ron Beyer

Comments

Ian A Davidson 5-May-13 20:15pm

This is wrong for C++. (You are probably thinking C#?)
"img" isn't a pointer to a LoadImage object.
"LoadImage img;" is creating a LoadImage object - it will create an object of LoadImage using the default constructor. Whether this has been implemented correctly to initialise the members is another matter, and I doubt it actually loads an image (unless there is some default specified) ;)
Regards,
Ian.

Angela2012 5-May-13 23:13pm

Thank you very much!

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Ian A Davidson · Accepted Answer · 2013-05-05T14:31:00

In the first place, don't access member variables of other classes directly. This is very bad practice. Your member variables should be private and you should declare methods (functions) to return the value. This prevents a caller from setting them to invalid values.

In the second place, why are the member variables "destroyed" when you reach that line? They shouldn't be. What is more likely is that you haven't initialised them.

You are using the default constructor to create the LoadImage object. Is this what you want, or do you want to specify a file path or a resource id, for example?

Do you have a default constructor that correctly initialises the member variables (probably to zero).

How about this, for example:

C++

class LoadImage
{
public:
    LoadImage()
    :   width(0),
        height(0),
        imgData(NULL)
    {
    }

    LoadImage
        (
        const std::string& filePath
        )
    :   width(0),
        height(0),
        imgData(NULL)
    {
        // load image from filePath into imgData and set height and width accordingly.
    }

    virtual ~LoadImage()
    {
        if (NULL != imgData)
        {
            // Free imgData according to how you initialised it
            // And set it back to null (not important in this case, but it is good practice)
            imgData = NULL;
        }
    }

    inline int GetHeight() const
    {
        return height;
    }

    inline int GetWidth() const
    {
        return width;
    }

    // ......

private:
    int width;
    int height;
    BYTE* imgData;
};