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(cpp-295.info)Cascaded Macros

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Cascaded Use of Macros
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   A "cascade" of macros is when one macro's body contains a reference
to another macro.  This is very common practice.  For example,

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE

   This is not at all the same as defining `TABLESIZE' to be `1020'.
The `#define' for `TABLESIZE' uses exactly the body you specify--in
this case, `BUFSIZE'--and does not check to see whether it too is the
name of a macro.

   It's only when you *use* `TABLESIZE' that the result of its expansion
is checked for more macro names.

   This makes a difference if you change the definition of `BUFSIZE' at
some point in the source file.  `TABLESIZE', defined as shown, will
always expand using the definition of `BUFSIZE' that is currently in
effect:

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE
     #undef BUFSIZE
     #define BUFSIZE 37

Now `TABLESIZE' expands (in two stages) to `37'.  (The `#undef' is to
prevent any warning about the nontrivial redefinition of `BUFSIZE'.)