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Declare variables at top of function or in separate scopes?

Asked 2010-09-22T20:33:39.940
52

Which is preferred, method 1 or method 2?

Method 1:

LRESULT CALLBACK wpMainWindow(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
    switch (msg)
    {
        case WM_PAINT:
        {
            HDC hdc;
            PAINTSTRUCT ps;

            RECT rc;
            GetClientRect(hwnd, &rc);           

            hdc = BeginPaint(hwnd, &ps);
            // drawing here
            EndPaint(hwnd, &ps);
            break;
        }
        default: 
            return DefWindowProc(hwnd, msg, wparam, lparam);
    }
    return 0;
}

Method 2:

LRESULT CALLBACK wpMainWindow(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
    HDC hdc;
    PAINTSTRUCT ps;
    RECT rc;

    switch (msg)
    {
        case WM_PAINT:
            GetClientRect(hwnd, &rc);

            hdc = BeginPaint(hwnd, &ps);
            // drawing here
            EndPaint(hwnd, &ps);
            break;

        default: 
            return DefWindowProc(hwnd, msg, wparam, lparam);
    }
    return 0;
}

In method 1, if msg = WM_PAINT when wpMainWindow function is called, does it allocate memory for all the variables on the stack at the beginning? or only when it enters the WM_PAINT scope?

Would method 1 only use the memory when the message is WM_PAINT, and method 2 would use the memory no matter what msg equaled?

c++ c function stack
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1 Answer

5

Since it's the compiler's job to optimize my code, and an hour of compiler-time is way cheaper than an hour of my time, and my time gets wasted if I need to scroll up and down the code to see where a variable was declared, I think my company wants me to keep everything as local as possible.

Not even am I talking about 'the smallest block', but 'as near to the place where it is used'!

LRESULT CALLBACK wpMainWindow(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) 
{ 
    switch (msg) 
    { 
        case WM_PAINT: 
        { 
            RECT rc; 
            GetClientRect(hwnd, &rc);            

            { // sometimes I even create an arbitrary block 
              // to show correlated statements.
              // as a side-effect, the compiler may not need to allocate space for 
              // variables declared here...
              PAINTSTRUCT ps; 
              HDC hdc = BeginPaint(hwnd, &ps); 
              // drawing here 
              EndPaint(hwnd, &ps); 
            }
            break; 
        } 
        default:  
            return DefWindowProc(hwnd, msg, wparam, lparam); 
    } 
    return 0; 
}
answered 2010-09-22T21:19:34.430

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