Why does this code with TCHAR and variadic arguments behave this way?

Question

I have the following helper function:

inline void DebugMessage(const TCHAR* fmtstr, ...)
{
        va_list args;
        va_start(args, fmtstr);

        TCHAR buffer[256];
        StringCbVPrintf(buffer, 256, fmtstr, args);
        OutputDebugString(buffer);

        va_end(args);
}

And I call it twice like so:

DebugMessage(_T("Test %d\n", 1)); // incorrectly closed _T()
DebugMessage(_T("Test %d\n"), 1); // correctly closed _T()

I get the following output:

Test 0
Test 1

The 2nd case works as expected. I am confused why the first case functions at all, rather than being an error?

score 3 · Accepted Answer · answered Sep 03 '13 at 03:02

3

_T is not a function, it's a macro that (in a Unicode build) expands to L ## x. The misplaced bracket doesn't cause a compile error, it simply changes which parts of the line gets consumed by the macro.

The macro only takes one parameter (x) and so in the first case, with the incorrect closure, the second parameter (1) is simply discarded, and the number you get in your output is simply a result of random data on the stack.

Note that by default, VS 2012 will issue a C4002 warning about this (too many actual parameters for macro) so you may want to check that you have warnings enabled properly.

answered Sep 03 '13 at 03:02

Jonathan Potter

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+1 Your preprocessor should issue a stern warning if incorrect parameter counts are supplied to a non-varadic macro (at least mine does) and outright error if warnings are configured to do so. – WhozCraig Sep 03 '13 at 03:05
Yeah I just added a note about that. – Jonathan Potter Sep 03 '13 at 03:05
Anyway to do this in Visual Studio 2010? – samoz Sep 03 '13 at 03:07
1

Be kind to your preprocessor, for she can be a heartless wench if you mistreat her =) – WhozCraig Sep 03 '13 at 03:07
The warning existed at least in VS 2008: http://msdn.microsoft.com/en-us/library/y37zb304(v=vs.90).aspx – Jonathan Potter Sep 03 '13 at 03:07
It's warning 4002. To treat it as an error, go to "Project Properties -> C/C++ -> Advanced -> Treat Specific Warnings As Errors" – samoz Sep 03 '13 at 03:15

Why does this code with TCHAR and variadic arguments behave this way?

1 Answers1