Programming in C: Unit I (d): Preprocessor Directives
#undef
with Example C Programs | Preprocessor Directives
As the name suggests, the #undef directive undefines or removes a macro name previously created with #define. Undefining a macro means to cancel its definition.
#UNDEF
As
the name suggests, the #undef directive
undefines or removes a macro name previously created with #define. Undefining a macro means to cancel its definition. This is
done by writing #undef followed by
the macro name that has to be undefined.
Like
definition, undefinition also occurs at a specific point in the source file,
and it applies starting from that point. Once a macro name is undefined, the
name of the macro ceases to exist (from the point of undefinition) and the
preprocessor directive behaves as if it had never been a macro name.
Therefore,
the #undef directive removes the
current definition of macro and all subsequent occurrences of macro name are
ignored by the preprocessor.
Note
If
you had earlier defined a macro with parameters, then when undefining that
macro you do not have to give the parameter list. Simply specify the name of
the macro.
You
can also apply the #undef directive
to a macro name that has not been previously defined. This can be done to
ensure that the macro name is undefined.
The
#undef directive when paired with a #define
directive creates a region in a source program in which the macro has a special
meaning. For example, a specific function of the source program can use certain
constants to define environment-specific values that do not affect the rest of
the program.
The
#undef directive can also be paired
with the #if directive to control conditional compilation of the source
program. We will read about the #if directive later in this section.
Consider
the following example in which the #undef directive removes definitions of a
symbolic constant and a macro.
#define MAX 10
#define MIN (X, Y) (((X) < (Y))
? ((X): (Y))
.
.
.
#undef MAX
#undef MIN
Programming in C: Unit I (d): Preprocessor Directives : Tag: : with Example C Programs | Preprocessor Directives - #undef
Programming in C: Unit I (d): Preprocessor Directives
Under Subject
Programming in C
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