Variable: ff-related-file-alist
ff-related-file-alist is an alias, customizable and buffer-local for
ff-other-file-alist, defined in find-file.el.gz.
Documentation
Alist of extensions to find given the current file's extension.
The value could be an alist or a symbol whose value is an alist. Each element of the alist has the form
(REGEXP (EXTENSION...))
where REGEXP is the regular expression matching a file's extension,
and EXTENSIONs is the list of literal file-name extensions to search
for. The list of extensions should contain the most used extensions
before the others, since the search algorithm searches sequentially
through each directory specified in ff-search-directories.
Alist elements can also be of the form
(REGEXP FUNCTION)
where FUNCTION is a function of one argument, the current file's name, that returns the list of possible names of the corresponding files, with or without leading directories. Note the difference: FUNCTION returns the list of file names, not their extensions. This is for the case when REGEXP is not enough to determine the file name of the other file.
If a file is not found, a new one is created with the first
matching extension or name (e.g., .cc yields .hh).
This alist should be set by the major mode.
Note: if an element of the alist names a FUNCTION as its cdr, that function must return a non-nil list of file-names. It cannot return nil, nor can it signal in any way a failure to find a suitable list of file names.