Using Void Function in C/C++ Windows API - c++

Question

I would just like to know how to implement a void function (which for instance returns a string) in a C/C++ GUI program and show the output in a window.
Would you put it in a WM_CREATE case in the WndProc function or under the WinMain function? Or is it more complicated, as in, you have to redirect the Command Prompt output to the window?
Edit:
So, I have, for example...
void function() {
cout << "Hello";
}
and then I want to implement this in a GUI...
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_CLOSE:
DestroyWindow(hwnd);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd, msg, wParam, lParam);
}
return 0;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow)
{
hwnd = CreateWindowEx(
WS_EX_CLIENTEDGE,
g_szClassName,
"The title of my window",
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT, 240, 120,
NULL, NULL, hInstance, NULL);
// Implement function somewhere here?
ShowWindow(hwnd, nCmdShow);
UpdateWindow(hwnd);
}

Answer 1

I don't think you need to go as low level as WndProc
The most basic way to implement a multiple return function in general is to pass by reference, for example
void someFunction(string* aS)
{ //do something
aS = someString;
}
It should be impemented something like this
string someString;
someFunction(&someString);
ShowMessage(someString);