OOP GUI, Template C++ - c++

Question

I'm a game designer coming from a Basic-like programming language and moving to C++.
For a new game I'd like to have a gui programmed using OOP.
My gui currently consists of a gui-class (the outer wrapper), a g_element-class (the middle wrapper containing all the common attributes) and a button-class (overriding and extending the g_element-class).
The problem is I'm getting the following error:
Severity Code Description Project File Line Suppression State
Error LNK2001 unresolved external symbol "public: static class
std::list,class std::allocator > > gui
::el_stack"(?el_stack#?$gui#V?$g_element#Vbutton######2V?$list#V?$g_element#Vbutton####V?$allocator#V?$g_element#Vbutton#####std###std##A)
I really think that this error is not the only problem - I might have a completely wrong approach for this whole thing. I'm also not really sure about that template-thing - I thought I'd be able to expand my g_element-class with gui-elements later (i.e. buttons, sliders, windows...) - but give me a hint if there's something that I can optimise.
Here is my gui.cpp file (so far):
using namespace std;
template <class GUI_ELEMENT> class gui {
protected:
void add_to_stack(GUI_ELEMENT elem) {
// The error comes from here..
// I wanted a list of all my g_elements (buttons)
el_stack.push_back(elem);
printf("size now: %d", el_stack.size());
}
unsigned int get_stack_size() {
return el_stack.size();
}
public:
static list<GUI_ELEMENT> el_stack; // The elements list
void render() {
// here, I'd like to iterate over all i.e. buttons to draw them
}
};
template <class GUI_ELEMENT> class g_element : public gui<g_element<GUI_ELEMENT> >{
private:
float x;
float y;
float w;
float h;
public:
void set_width(float width) {
this->w = width;
}
float get_width() {
return this->w;
}
};
class button : public g_element<button> {
protected:
char* caption;
public:
button(float x, float y, float w, float h, char* caption) {
this->set_width(w);
this->set_caption(caption);
this->add_to_stack(*this);
}
void set_caption(char* caption) {
this->caption = caption;
}
char* get_caption() {
return this->caption;
}
};
I'd like to use my gui like so:
// Create a few test buttons
button b1(50, 50, 100, 50, "Test");
b1.set_width(150);
float s = b1.get_width();
printf("size w: %f", s);
printf("\ncaption: %s", b1.get_caption());
button b2(50, 50, 100, 50, "Test");
button b3(50, 50, 100, 50, "Test2");
button b4(50, 50, 100, 50, "Test3");
// Rendering currently (all buttons at once)
gui<button> G;
G.render();
// but this would be nicer:
gui::render_buttons()
// or
gui<button>::render()
Is there somebody who might help me? Really big thanks in advance!

Answer 1

All static fields should be defined outside of the class definition. In your case you have to add these lines of code after a class definition:
template<class GUI_ELEMENT>
list<GUI_ELEMENT> gui<GUI_ELEMENT>::el_stack;