Q: Is the abbreviated pointer comparison ``if(p)'' to test for non-null pointers valid? What if the internal representation for null pointers is nonzero?
A: It is always valid.
When C requires the Boolean value of an expression, a false value is inferred when the expression compares equal to zero, and a true value otherwise. That is, whenever one writes
if(expr)where ``expr'' is any expression at all, the compiler essentially acts as if it had been written as
if((expr) != 0)Substituting the trivial pointer expression ``p'' for ``expr'', we have
if(p) is equivalent to if(p != 0)and this is a comparison context, so the compiler can tell that the (implicit) 0 is actually a null pointer constant, and use the correct null pointer value. There is no trickery involved here; compilers do work this way, and generate identical code for both constructs. The internal representation of a null pointer does not matter.
The boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to (expr)?0:1 or to ((expr) == 0)which leads to the conclusion that
if(!p) is equivalent to if(p == 0)
``Abbreviations'' such as if(p), though perfectly legal[footnote] , are considered by some to be bad style (and by others to be good style; see question 17.10).
See also question 9.2.
K&R2 Sec. A7.4.7 p. 204
ISO Sec. 126.96.36.199, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 188.8.131.52, Sec. 6.6.5
H&S Sec. 5.3.2 p. 122