Thread: __va_args__

Originally Posted by syzygy

Code:

void DebugPrint( const char* kpFormatString, ... );
#define PRINT(... ) { DebugPrint( __VA_ARGS__ ); }

if( rc < 0 )
{
	PRINT( "blah blah 0x%x\n", rc );
}
else
{
	PRINT("glub glub\n");
}

Why do I need {} around my PRINT calls? If I don't place them there, I get:

Code:

error: expected primary-expression before 'else'

I don't understand why it's trying to read in parameters past the delimited area.

This is using VS2005.

Because you are writing a semicolon at the end of the PRINT() macro invokation. Without braces, this semicolon serves as an empty statement, which terminates the if-statement, and causes the following else to become illegal. Change you definition of PRINT to this:

Code:

#define PRINT(... ) do { DebugPrint( __VA_ARGS__ ); } while(0)

Now, the trailing semicolon will attach to the do-while, and everything should work.

After macroexpansion, the code looks like this:

Code:

if (rc < 0)
{
    { DebugPrint("blah blah 0x%x\n", rc); };
}
else
{
   { DebugPrint("glub glub\n"); };
}

Now, if you remove the surrounding braces, it looks like this:

Code:

if (rc < 0)
    { DebugPrint("blah blah 0x%x\n", rc); };
else
   { DebugPrint("glub glub\n"); };

See the problem? The bolded semicolon effectively ends the "if" expression and you're left with a mismatched "else". Thus, you need the braces... or you could cut out the braces surrounding the `DebugPrint' call in the `PRINT' macro.