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#define/#undef
Define/Undefine Constant Directives
Syntax
#define
name[(arg, ... ,arg)] token-string
#undef
name
Description
The preprocessor supports two directives for defining and undefining con-
stants:
Example
B-2
•
The #define directive assigns a string to a constant.
Subsequent
occurrences of
name
are replaced by
token-string.
The
name
can be
immediately followed by an argument list; the arguments are sepa-
rated by commas, and the list is enclosed in parentheses. Each oc-
currence of an argument is replaced by the corresponding set of
tokens from the comma-separated string.
When a macro with arguments is expanded, the arguments are placed
into the expanded
token-string
unchanged. After the entire
token-
string
is expanded, the preprocessor scans again for names to expand
at the beginning of the newly created
token-string,
which allows for
nested macros.
Note that there is no space between
name
and the open parenthesis
at the beginning of the argument list. A trailing semicolon is not re-
quired; if used, it is treated as part of the
token-string.
•
The #undef directive undefines the constant
name;
that is, it causes
the preprocessor to forget the definition of
name.
The following example defines the constant
f:
#define f(a,b,c) 3*a+b-c
The following line of code uses the definition of
f:
f(27,begin,minus)
This line is expanded to:
3*27+begin-minus
To undefine
f,
enter:
#undef
f
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