BCB: how to get the (approximate) width of a character in a given TFont?

Question

It's a TMemo, not that that should make any difference.

Googling suggests that I can use Canvas->TextWidth() but those are Delphi examples and BCB doesn't seem to offer this property.

I really want something analogous to memo->Font->Height for width.

I realize that not all fonts are fixed width, so a good estimate will do.

All that I need is to take the width of a TMemo in pixels and make a reasonable guess at how many characters of the current font it will hold.

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Of course, if I really want to be lazy, I can just google for the average height/width ratio, since height is known. Remember, an approximation is good enough for me if it is tricky to get exact.

http://www.plainlanguagenetwork.org/type/utbo211.htm says, " A width to height ratio of 3:5 (0.6) is recommended for most applications"

Answer 1

Actually your google search is not entirely off. You do need access to a canvas object, or at least a handle to a DC object. In general when searching for help concerning VCL classes it often pays to search for delphi examples since these are more common.

Anyway to calculate the size of a string you could have a look at the TextExtent function, it is a function for the TCanvas class. Simply pass the character which width you want to test, and the return value will be a TSize construct. However there is also a TextWidth function, as well as a TextHeight function. You can use these as well. Actually these call the TextExtent internally.

You have to note one thing though, the functions use the current font of the TCanvas object, more specifically the font bound to the DC the canvas uses. So assign the font you wish to test with first, and then pass the character.

I have some old code that calculates the width of a string like this:

// This canvas could be the form canvas: canvas = Form1->Canvas or the 
// memo canvas which will probably be what you want.
canvas->Font->Assign(fontToTest);
int textwidth = TextWidth(textToTest);

If you want more control of what to do, you can also do this using the Windows API, this is essentially what the VCL does for you, in that case the following example would look like this:

// This canvas could be the form canvas: canvas = Form1->Canvas
canvas->Font->Assign(fontToTest);
// The initial size, this is really important if we use wordwrapping. This is 
// the text area of the memo control.
TRect rect = ClientRect; 
// This is the font format we wish to calculate using, in this example our text
// will be left aligned, at the top of the rectangle.
fontformat = DT_LEFT | DT_TOP;
// Here we calculate the size of the text, both width and height are calculated
// and stored in the rect variable. Also note that we add the DT_CALCRECT to the
// fontformat variable, this makes DrawTextEx calculate the size of the text,
// without drawing it.
::DrawTextEx(canvas->handle, 
            textToTest.c_str(), 
            textToTest.Length(), 
            &rect, 
            fontformat | DT_CALCRECT, 
            NULL);
// The width is:
int width = rect.Width();

The fontformat is a parameter that specifies different options for how to align and layout the text, if you plan on drawing text it will be a good idea to check out the different possibilities it offers: DrawTextEx Function [1]

EDIT: Reading through your question again, it struck me that the function you might be searching for is: GetTextExtentExPoint Windows API documentation states the following about this function:

The GetTextExtentExPoint function retrieves the number of characters in a specified string that will fit within a specified space and fills an array with the text extent for each of those characters. (A text extent is the distance between the beginning of the space and a character that will fit in the space.) This information is useful for word-wrapping calculations.

You can find more information about the GetTextExtentExPoint function here: GetTextExtentExPoint Function [2]

[1] http://msdn.microsoft.com/en-us/library/dd162499%28VS.85%29.aspx
[2] http://msdn.microsoft.com/en-us/library/dd144935%28VS.85%29.aspx

Answer 2

What you could do, if you have access to Win32 API functions, is create a RichEdit Window the same size as your TMemo window, place the text in the RichEdit window, send the EM_FORMATRANGE message to the window and from the result determine how many characters it will hold. Of course this method will work with multiple lines etc...