Mein Code:

class window {

		HINSTANCE hInstance_;
		HWND hWnd_;
		std::string classname_, caption_;

/*...*/
		window(HINSTANCE hi, const char* cn, const char* cap, WNDPROC pf);
/*...*/}

window::window(HINSTANCE hi, const char* cn, const char* cap, WNDPROC pf)
:hInstance_(hi), hWnd_(0), classname_(cn), caption_(cap) {
	WNDCLASSA wndclass;
	wndclass.style         = CS_HREDRAW | CS_VREDRAW;
	wndclass.lpfnWndProc   = pf;
	wndclass.cbClsExtra    = 0;
	wndclass.cbWndExtra    = 0;
	wndclass.hInstance     = hInstance_;
	wndclass.hIcon         = LoadIcon(0, IDI_APPLICATION);
	wndclass.hCursor       = LoadCursor(0, IDC_ARROW);
	wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
	wndclass.lpszMenuName  = 0;
	wndclass.lpszClassName = classname_.c_str();
	if (!RegisterClassA(&wndclass)) {
		/*...*/
		return;
	}
	hWnd_ = CreateWindowA(classname_.c_str(),	// window class name
						caption_.c_str(),		// window caption
						WS_OVERLAPPEDWINDOW,	// window style
						0,						// initial x position
						0,						// initial y position
						CW_USEDEFAULT,			// initial x size
						CW_USEDEFAULT,			// initial y size
						0,						// parent window handle
						0,						// window menu handle
						hInstance_,				// program instance handle
						0);						// creation parameters
	if (!hWnd_) {
		/*...*/
		return;
	}
}

Woran liegt das Problem?

Und was kommt am anderen Ende raus?

Etwas wie kleine Quadraten.