This is not a compiler error, it is a preprocessor error. It occurs when the preprocessor encounters invalid syntax while trying to evaluate an expression in a #if
or #elif
directive.
One common cause is the sizeof
operator in an #if
directive:
For example:
#define NBITS (sizeof(TYPE)*8)
//later
#if (NBITS>16) //ERROR
This is an error because sizeof
is evaluated by the compiler, not the preprocesor.
Type casts are also not valid preprocessor syntax:
#define ALLBITS ((unsigned int) -1)
//later
#if (ALLBITS>0xFFFF) //ERROR
The rules for what can be in a valid expression are here.
Note also that #if
will evaluate an undefined macro as 0, unless it looks like it takes arguments, in which case you also get this error:
So if THIS
is undefined:
#if THIS == 0 //valid, true
#if THIS > 0 //valid, false
#if THIS() == 0 //invalid. ERROR
Typos in your #if
statement can also cause this message.