C++ Winapi - wrapping up a callback of subclass in a class - c++

Question

I wanted to create a class that I will be used to create a static control. The problem is when subclassing the control, the callback should be static which means I won't be able to access non-static variables and function inside the callback.
I've been pulling my hair to make my code works but still no success.
MyClass.cpp
#include "MyClass.h"
MyClass::MyClass(){
non_static_variable = 0; //default is zero.
}
LRESULT CALLBACK MyClass::SubClassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData) {
MyClass* pThis = reinterpret_cast<MyClass*>(dwRefData);
char buffer[5];
sprintf_s(buffer, "The value of non_static_variable is: %d \n", pThis->non_static_variable);
OutputDebugStringA(buffer);
switch (uMsg) {
case WM_PAINT: {
// do nothing for now
return 0;
}
case WM_NCDESTROY: {
RemoveWindowSubclass(hwnd, SubClassProc, uIdSubclass);
break;
}
}
return DefSubclassProc(hwnd, uMsg, wParam, lParam);
}
void MyClass::CreateStaticControl(HWND WindowHandle) {
StaticControl = CreateWindowEx(0, L"STATIC", NULL, WS_CHILD | WS_VISIBLE | SS_OWNERDRAW, x, y, width, height, WindowHandle, NULL, NULL, this);
SetWindowSubclass(StaticControl, &SubClassProc, ID, reinterpret_cast<DWORD_PTR>(this));
}
void MyClass::SetValue(int value){
non_static_variable = value; //test if I can access this in callback
}
MyClass.h
#include "header.h"
public:
MyClass();
void CreateStaticControl(HWND window);
void SetValue(int value);
static LRESULT CALLBACK SubClassProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData);
private:
int non_static_variable;
HWND StaticControl;
};
Then call the class inside my WM_CREATE from my main window procedure, I used it like this.
...
case WM_CREATE:{
MyClass myClass;
myClass.CreateStaticControl(hwnd);
myClass.SetValue(888);
break;
}
You might think that this is a duplicate of others, I found a lot of them but since this is about subclassing and I don't have access to WM_NCCREATE I can't properly set the pointer of my class. Can anyone help me?
EDIT: I put the exact code I have.

Answer 1

You can use the dwRefData parameter of SetWindowSubclass() to pass the this pointer to the dwRefData parameter of the callback. You can then type-cast that parameter to access non-static members. No need to use GWLP_USERDATA at all (especially since you are not using the callback's lParam correctly when setting GWLP_USERDATA).
Try this instead:
MyClass.h
class MyClass {
public:
MyClass();
void CreateStaticControl(HWND window);
static LRESULT CALLBACK SubClassProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData);
private:
int non_static_variable;
HWND StaticControl;
};
MyClass.cpp:
void MyClass::CreateStaticControl(HWND WindowHandle) {
StaticControl = CreateWindowEx(0, L"STATIC", NULL, WS_CHILD | WS_VISIBLE | SS_OWNERDRAW, x, y, width, height, WindowHandle, NULL, NULL, this);
SetWindowSubclass(StaticControl, &SubClassProc, ID, reinterpret_cast<DWORD_PTR>(this));
}
LRESULT CALLBACK MyClass::SubClassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData) {
MyClass* pThis = reinterpret_cast<MyClass*>(dwRefData);
switch (uMsg) {
case WM_PAINT: {
// use pThis->non_static_variable as needed...
return 0;
}
case WM_NCDESTROY: {
RemoveWindowSubclass(hwnd, SubClassProc, uIdSubclass);
break;
}
}
return DefSubclassProc(hwnd, uMsg, wParam, lParam);
}