What is the scope of #undef ?

I don't know whether I should be proud or ashamed to say that I've never in my whole life needed to use #undef - until today. :o but what is the 'scope' of a #undef statement (i.e. how long does it last for, if it appears in a source file, as distinct from a header file?)

Let's say I'm compiling a project with twenty cpp source files. I have a preprocessor definition - let's say, #define MY_DEFINE "Hello World" which I've set up in my IDE.

In module 5 (out of the twenty modules) I need to #undef MY_DEFINE for some reason. Is it then #undef'd until the compiler finishes compiling or is it only #undef'd for that one source file?

In other words, when the compiler reaches the 6th source file, will it re-establish #define MY_DEFINE "Hello World" :confused:

Remeber that compiler processes each translation unit separately. If you undef some defined sybol, and afterwards compile another file with the same compiler settings, symbol will be defined again.
Each define works from place of occurence to the end of translation unit. The same is for undef, unless symbol is redefined.
Anyway, playing with macros is risky, and playing this way seems to be more than risky :)