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comp.lang.c FAQ list · Question 5.3

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

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.

References: K&R2 Sec. A7.4.7 p. 204
ISO Sec., Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec., Sec. 6.6.5
H&S Sec. 5.3.2 p. 122

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