Calling a function to create a WinAPI button doesn't do anything - c++

Question

After a while experimenting with WinAPI, I got a window with a button working. The code was put into my window procedure and looked like this:
std::vector<HWND> buttons;
HWND window;
LRESULT CALLBACK CSwindowProc(HWND window, unsigned int message, WPARAM shortParam, LPARAM longParam) {
switch (message) {
case WM_CREATE:
buttons.push_back(
CreateWindow(_T("BUTTON"), _T("OK"), WS_TABSTOP | WS_VISIBLE | WS_CHILD | BS_DEFPUSHBUTTON,
10, 10, 100, 100, window, NULL, (HINSTANCE)GetWindowLongPtr(window, GWL_HINSTANCE), NULL)
);
break;
case WM_DESTROY: PostQuitMessage(0); break;
default: return DefWindowProc(window, message, shortParam, longParam); break;
}
return 0;
}
Please note "buttons" is a HWND vector and "window" is just an HWND initialized with CreateWindow. Context to these snippets are identical to the code at the end of this question.
To simplify what I have, I decided to move the button creation code into a new function (CSnewButton). Then, I removed the button creation code from the window procedure and added a call to CSnewButton(). At this point the code looked like this:
std::vector<HWND> buttons;
HWND window;
void CSnewButton() {
buttons.push_back(
CreateWindow(_T("BUTTON"), _T("OK"), WS_TABSTOP | WS_VISIBLE | WS_CHILD | BS_DEFPUSHBUTTON,
10, 10, 100, 100, window, NULL, (HINSTANCE)GetWindowLongPtr(window, GWL_HINSTANCE), NULL)
);
}
LRESULT CALLBACK CSwindowProc(HWND window, unsigned int message, WPARAM shortParam, LPARAM longParam) {
switch (message) {
case WM_CREATE:
CSnewButton();
break;
case WM_DESTROY: PostQuitMessage(0); break;
default: return DefWindowProc(window, message, shortParam, longParam); break;
}
return 0;
}
Surprisingly, the button was no longer present on the window. By simply moving the code from the procedure to a function called by the procedure, the button disappeared.
I quickly did some debugging to rule out some problems. I added a message box to the function, and the message box did appear, so the function was being called. The C++ inline modifier didn't help my case either once added to the CSnewButton function. I see no errors or warnings on either compile time or run time either.
So I guess I either have a misunderstanding of C++ or aggressive C++ compilers (I wouldn't be surprised), a misunderstanding of WinAPI (probably the culprit) or something is so very incredibly obviously wrong with my code (I wouldn't be surprised either). Anyway, I would like to know why moving the button creation code into a function takes away the code's function, and a solution to this problem (if possible).
Full code showcasing the problem:
#include <windows.h>
#include <tchar.h>
#include <vector>
HINSTANCE instance;
HWND window;
std::vector<HWND> buttons;
//This function inexplicably does not create the button, however, if you were
//to take it's contents and paste them over a call to this function, it works
//fine.
void CSnewButton() {
buttons.push_back(
CreateWindow(_T("BUTTON"), _T("OK"), WS_TABSTOP | WS_VISIBLE | WS_CHILD | BS_DEFPUSHBUTTON,
10, 10, 100, 100, window, NULL, (HINSTANCE)GetWindowLongPtr(window, GWL_HINSTANCE), NULL)
);
}
LRESULT CALLBACK CSwindowProc(HWND window, unsigned int message, WPARAM shortParam, LPARAM longParam) {
switch (message) {
case WM_CREATE:
CSnewButton(); // Issue Here
break;
case WM_DESTROY: PostQuitMessage(0); break;
default: return DefWindowProc(window, message, shortParam, longParam); break;
}
return 0;
}
void CScreateWindow() {
WNDCLASSEX cclass = {
sizeof(WNDCLASSEX), CS_VREDRAW | CS_HREDRAW,
CSwindowProc, 0, 0, instance, NULL, LoadCursor(NULL, IDC_ARROW),
(HBRUSH)(COLOR_WINDOW + 1), NULL, _T("MyClass"), NULL
};
if (!RegisterClassEx(&cclass)) {
MessageBox(NULL, _T("CSControlApp failed to register window."), _T("CSControlApp failure"), MB_OK | MB_ICONERROR); return;
}
window = CreateWindow(cclass.lpszClassName, _T("My Window"), WS_OVERLAPPEDWINDOW ^ WS_THICKFRAME,
CW_USEDEFAULT, CW_USEDEFAULT, 600, 600, NULL, NULL, instance, NULL);
if (!window) {
MessageBox(NULL, _T("CSControlApp failed to create window."), _T("CSControlApp failure"), MB_OK | MB_ICONERROR); return;
}
}
int CALLBACK WinMain(HINSTANCE h, HINSTANCE p, LPSTR cmd, int show) {
instance = h;
CScreateWindow();
ShowWindow(window, show);
UpdateWindow(window);
MSG msg;
while (GetMessage(&msg, NULL, 0, 0)) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}

Answer 1

This is a C language issue. You have two separate variables with the same name, and you're confusing them.
HWND window; // This is a global variable
LRESULT CALLBACK CSwindowProc(HWND window, ...) // This is a parameter
Inside CSWindowProc when you say window you get the parameter. Outside CsWindowProc when you say window you get the global variable, which hasn't yet been initialized at the point at which you're trying to use it. (The call to CreateWindow in CScreateWindow hasn't returned yet.)
You can resolve the problem by giving CSnewButton an argument:
void CSnewButton(HWND window)
and calling it accordingly:
case WM_CREATE:
CSnewButton(window);
break;
To avoid any similar confusion in future, it would probably be best to remove the global variable altogether.