Question

In c extensions to ruby, to call a method you can do

rb_funcall(object, rb_intern("method name"), argumentcount, arg1, arg2, …);

where rb_intern() returns some internal represantation of the method name.
I have seen some code which does instead

ID method;
CONST_ID(method, "method name");
rb_funcall(object, method, argumentcount, arg1, arg2, …);

What exactly is the difference beetwen rb_intern() and CONST_ID. Which advantages does CONST_ID() have?

Answer 1

The CONST_ID macro calls rb_intern2 (which is about the same as rb_intern) to get the ID, but there is one big difference. If you look at the CONST_ID macro source in include/ruby/ruby.h, you’ll see that it starts a new block and defines a static ID variable to cache the result. If, next time that block is executed, the static variable is already set, it just returns the cached result instead of searching for the ID all over again.

So they do the same thing, but CONST_ID should be faster for multiple lookups of the same string.