Macro-based Substitutions in Source Code

A friends asks: "How can one easily replace a method call (which can contain arguments with brackets in its invocation code) with a simple field access?

For example, we might want to substitute

p->getname(a, strlen(a))

p->name

It is not possible to use a regular expression in our editor, because we can't account for the nested brackets in the method call's arguments. It turns out that a simple pipeline involving sed and the C preprocessor cpp will do the job for us. The trick is to hide the other C preprocessor commands, instruct the preprocessor to pass comments unchanged, and not emmit #line directives. The following Unix command will do the trick:

sed 's/#/^A/g;s/\\/^B/g;1i\
#define getname(a, b) name' oldfile.c |
cpp -P -C |
sed 's/^A/#/g;s/^B/\\/g' >newfile.c

We also can use the same idea to:

• change the order of a function's arguments, or
• add or remove arguments to a function.

However, the substitution takes place at the program's lexical level, and does not take into account types and scope. For that we need a more powerful tool, like CScout (in the case of C code), or the refactoring capabilities of Eclipse (in the case of Java code).

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