Function parameter ...

[timmay3141]

I was looking at some code in a Direct3D tutorial and ran across the following function used for debugging:

void WriteToLog(char *lpszText, ...)
{
if(m_fEnableLogging)
{
va_list argList;
FILE *pFile;

//Initialize variable argument list
va_start(argList, lpszText);

//Open the log file for appending
pFile = fopen("log.txt", "a+"

//Write the text and a newline
vfprintf(pFile, lpszText, argList);
putc('\n', pFile);

//Close the file
fclose(pFile);
va_end(argList);
}
}

I thought I knew C++ pretty well, but none of the books I've ever used have mentioned using ... as a function parameter. I'd seen it before in printf too, but I'd just ignored it. Anyway, can someone tell me what ... does and how it is being used?

 

[Zyrenthian]

it is called an elipsis i believe. The Stroustrup book covers it and most c++ books do as well. The only problem is you need to know what it is called. The first param usually describes how many args there are and their types (if applicable). then with the va_start/end you can look through them.

Hope this is of some assistance.

Matt

 

[timmay3141]

I think I got it. I'd seen the ellipsis before, but only in catch blocks. I found stuff about va_start/end in a c++ reference book, but my original c++ book doesn't mention it (it wasn't very good). Thanks for the help.

 

[apatterno]

Yeah, it is the same syntax used by the C library's printf, sprintf, and fprintf.

 

[rossno]

It's called a variadic argument list, which means "any number or typeof arguments." ... from Bruce Eckel's book "C++ Inside and Out" (out of print now). It might still be in his newer book "Thinking in C++".

http://www.mindview.net/Books/TICPP/ThinkingInCPP2e.html

Gook Luck,

 

[RbgoWeb]

You can easely set it up yourself without the va_arg macros.
e.g.
long Add(long argc, ...)
{
long* argv = &argc + 1;
long total=0, i=0;

while(i < argc)
total += argv[i++];

return total;
}

 

[mingis]

Binaryshi, your method is architecture-dependent, because some machines have upwards growing stack.

 

[RbgoWeb]

Thanks a lot mingis, I didn't know that.
Do you know what machines have this upwards growing stack?