Question

Currently I’m working on a GUI system, and I’ve made a GUIButton object which has this inheritance structure:

GUIObject
    |
GUIImage
    |
GUIButton

Where GUIObject is a base class with some virtual functions (render, update, etc).

Anyway, because of the way I want to handle callbacks for the GUI system, GUIButton is a template class, where T is the object that has a method that should be called back.

Here’s the header file:

 #include "GUIImage.h"

template<class CallbackClass> 
class GUIButton : public GUIImage
{
public:

    // Constructor //
    GUIButton();

    // GUIObject Overides //

    bool update(const GUIUpdateInfo& updateInfo);
    void update();

    void render(const GUIRenderInfo& renderInfo);

    string getType();

    // Callback //

    void setCallback(void (CallbackClass::* callback)(), CallbackClass* instance);

    // Destructor //
    ~GUIButton();

private:

    void (CallbackClass::*  m_callback)();
    CallbackClass*          m_callTo; 
};

You can see that their is also the function setCallBack, which sets the method from the CallbackClass and the particular instance to be used.

Anyway, after all this, my code compiles fine but the moment I try to use the new class I get the following linking error:

    Undefined symbols:
  "GUIButton<GUIAppTest>::GUIButton()", referenced from:
      GUIAppTest::GUIAppTest()in GUIAppTest.o
      GUIAppTest::GUIAppTest()in GUIAppTest.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

I’m working with Xcode as an IDE, so the error format might be slightly different then normal.

The code that uses the class is like so:

GUIButton<GUIAppTest>* newButton = new GUIButton<GUIAppTest>();

newButton->loadImage("Data/Sketch.png", 1);
newButton->setTextureCoordinates(0.0, 0.0, 1.0, 1.0);
newButton->setLocationCoordinates(0.0, 0.0, 84, 84);

newButton->setCallback(&GUIAppTest::onClick, this);

m_objects.push_back(newButton);

The problem is probably something very simple, but I’ve been stuck here for a while now so I thought I might as well get some help. The use of GUIButton actually occurs within the implementation of GUIAppTest – the class that is passed as a template to GUIButton – Would might this be the cause of my problem?

I’m certain that all my functions have been implemented as well, so the problem shouldn’t lie there.

If anyone could give me some pointers, I’d much appreciate it!

Answer 1

Define constructor

template<class CallbackClass>  
class GUIButton : public GUIImage 
{ 
public: 

    // Constructor // 
    GUIButton() : m_callTo(), m_callback() {} // <---- here