Thread: Using GetTextExtentPoint32 to get char width

Hi,

I'm trying to output a single character, get that character's width, and then increase a variable by that amount. Currently, my code looks like this:

In MainWndProc:

Code:

    static char ch[2];
    LPSTR ch1;
    SIZE sz;
    size_t * pcch;

Then, in WM_PAINT:

Code:

                TextOut(hdc, PosX, PosY * dwCharY, ch, 1);
                ch1 = ch;
                StringCchLength(ch1, 11, pcch);
                GetTextExtentPoint32(hdc, ch1, *pcch, &sz);
                PosX += sz.cx;

Unfortunately when I run the program, it crashes after 1 character is output. Anyone got any ideas as to how I can increase the text's x position without having it crash? I'm pretty new to this stuff.

Thanks.

Originally Posted by Sebastiani

>> Unfortunately when I run the program, it crashes after 1 character is output. Anyone got any ideas as to how I can increase the text's x position without having it crash?

You really need to get a firm grasp on pointers. It's not a chunk of memory but rather an *address* to some chunk of memory (supplied by you). Incidentally, you don't necessarily need to even *declare* a pointer - simply pass the address of the memory directly to whatever function you're using (StringCchLength, for instance).

Finally, if you insist on using such a wacky API as StringCchLength, you need to at least use it correctly. It returns an HRESULT that needs to be checked with the SUCCEEDED macro, and the length you pass to it should not, of course, exceed the size of the string (I'm not say it will necessarily fail, but it is incorrect nonetheless).

Thanks for the reply. So if StringCchLength isn't suitable, what would be? To be honest I just used it like that because they did so on msdn here:

Drawing Text from Different Fonts on the Same Line (Windows)

About the pointers- in that example, they just declare size_t * pcch; and then proceed to put it into StringCchLength. Do you need to do something else to do it before that?

Originally Posted by Sebastiani

>> Thanks for the reply. So if StringCchLength isn't suitable, what would be? To be honest I just used it like that because they did so on msdn here:

Well, it *is* generally a bad idea to incorporate random pieces of code into your programs. Now you see why. If you see a code example somewhere, the best thing to do is dissect it, find out what it does, and only when you fully understand the implications of using it should you add it to your code. In this case, StringCchLength is similar to strlen/wcslen (or more appropriately the strnlen/wcsnlen extensions), and IMO complete overkill. To your credit, looking at the MSDN example, it looks like whoever put together the sample screwed up. That's pretty funny, but I'm actually not suprised - I've also seen MSDN examples that use void main! So perhaps it would be a good idea to be weary of code samples posted on MSDN. At any rate, in this case you know that the string being passed to GetTextExtentPoint32 has a length of 1, so there really isn't any need for StringCchLength or pcch at all.

>> About the pointers- in that example, they just declare size_t * pcch; and then proceed to put it into StringCchLength. Do you need to do something else to do it before that?

The correct way would have been to declare a size_t variable and pass the address of it to the function.

Right, cool. Thanks for that. That all makes sense. I have read/ understood stuff about pointers before, and used them in some console-based programs. What you've said/ suggested there goes along with what I've read.

The thing is, often with more complicated programs (ie this Windows stuff) it seems like they'll introduce stuff that works differently to the stuff in my C++ book. Hence that was why I was just like "Screw it I'll just try what they wrote even though it doesn't seem to make sense.".

Originally Posted by novacain

Code:

size_t            sz;

TextOut(hdc, PosX, PosY * dwCharY, ch, 1);
GetTextExtentPoint32(hdc, ch, 1, &sz); //you know the number of chars in the string
PosX += sz.cx;

or use lstrlen() to find the string length.

Also look at DrawText() and CALC_RECT
(first call to find out the rect req and then call again to write the text, using the rect to position correctly).

I tried using DrawText in my example program. Basically what I'm aiming at is a program that will let me output text to a window the same way I can with the console (ie text that will just go to the next line when it reaches the edge of the window etc.). I tried using DrawText in the following way, but it didn't work. It just drew a single, weird looking character in the rectangle. Again, I tried to base it on stuff from msdn. Do you know why it doesn't work?

Also, I looked up CALC_RECT but couldn't find anything other than something this guy wrote for his own program:

Problem with function - C++ Forums

Anyway thanks again for the help, and here's my current code:

Code:

    static char ch[2];                  // current character 
    static char output_line[500];
    static PTCHAR pchOutputBuf; // input buffer 
    static string outputbuf;
    static int PosX = 0;
    static int PosY = 0;

Code:

    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);
        PosX = 0;
        PosY = 0;
        SetRect(&rc, 200, 200, 300, 300);
        for (int b = 0; b < 2; b++) {
            for (int a = 0; a < strlen(output_line); a++)
                ch[0] = 00;
            for (int a = 0; a < outputbuf.size(); a++) {
                ch[0] = outputbuf[a];
                TextOut(hdc, PosX, PosY * dwCharY, ch, 1);
                ch1 = ch;
                DrawText(hdc, ch1, 1, &rc, DT_LEFT);
                GetTextExtentPoint32(hdc, ch1, 1, &sz);
                PosX += sz.cx;
                if ((DWORD) PosX > dwLineLen) { 
                    PosX = 0;
                    ++PosY; 
                } 
            }
        }
        EndPaint(hwnd, &ps);
        return 0;

and

Code:

    case WM_TIMER:
        //process the timer msgs, check which timer has fired
        switch (wParam)
            case IDT_TIMER1:
            {
                hdc = GetDC(hwnd); 
                outputbuf += "Hello world. This is a long string to fill up the window. ";
                ReleaseDC(hwnd, hdc);
                InvalidateRect(hwnd, NULL, TRUE); 
                return 0;
		    }
        return 0;

Thanks for your help.