c++, cscope, ctags, and vim: Finding classes that inherit from this one

Question

In a rather large code base with a few layers is there a way in vim or from the command line to find all classes that are derived from a base class? grep is an option but can be slow since grep does not index.

Answer 1

Neither cscope nor ctags allow us to deal with inheritance directly but it's relatively easy to work around that limitation because derived classes are also indexed.

cscope

In cscope, looking for "C symbol" Foobar usually lists the original class and classes inheriting from it. Since the search is done against a database, it is lightning fast.

Alternatively, you could use cscope's egrep searching capabilities with a pattern like :.*Foobar to list only classes inheriting from Foobar.

So, even if we don't have a dedicated "Find classes inheriting from this class" command, we can get the work done without much effort.

ctags

While ctags allows you to include inheritance information with --fields=+i, that information can't be used directly in Vim. The inherits field is parsed by Vim, though, so it might be possible to build a quick and dirty solution using taglist().

ack, ag

Those two programs work more or less like grep but they are targeted toward searching in source code so they are really faster than grep.

In my Vim config, :grep is set to run the ag program instead of the default grep so, searching for classes derived from the class under the cursor would look like:

:grep :.*<C-r><C-w><CR>

Here are the relevant lines from my ~/.vimrc:

if executable("ag")
  set grepprg=ag\ --nogroup\ --nocolor\ --ignore-case\ --column
  set grepformat=%f:%l:%c:%m,%f:%l:%m
endif

Answer 2

If you build your tags files with Exuberant CTags using inheritance information (see the --fields option), then the following script will work. It adds an :Inherits command which takes either the name of a class (e.g. :Inherits Foo) or a regular expression.

Like the :tag command, you indicate that you want the search with a regex by preceding it with a '\' character, e.g. :Inherits \Foo.*.

The results are put into the window's location list, which you browse with :ll, :lne, :lp, etc. VIM doesn't seem to allow scripts to modify the tag list which is what I'd prefer.

If you're wondering why I don't use taglist() for this, it's because taglist() is incredibly slow on large tag files. The original post had a version using taglist(), if you're curious you can browse the edit history.

" Parse an Exuberant Ctags record using the same format as taglist()
"
" Throws CtagsParseErr if there is a general problem parsing the record
function! ParseCtagsRec(record, tag_dir)
    let tag = {}

    " Parse the standard fields
    let sep_pos = stridx(a:record, "\t")
    if sep_pos < 1
        throw 'CtagsParseErr'
    endif
    let tag['name'] = a:record[:sep_pos - 1]
    let tail = a:record[sep_pos + 1:]
    let sep_pos = stridx(tail, "\t")
    if sep_pos < 1
        throw 'CtagsParseErr'
    endif
    " '/' will work as a path separator on most OS's, but there
    " should really be an OS independent way to build paths.
    let tag['filename'] = a:tag_dir.'/'.tail[:sep_pos - 1]
    let tail = tail[sep_pos + 1:]
    let sep_pos = stridx(tail, ";\"\t")
    if sep_pos < 1
        throw 'CtagsParseErr'
    endif
    let tag['cmd'] = tail[:sep_pos - 1]

    " Parse the Exuberant Ctags extension fields
    let extensions = tail[sep_pos + 3:]
    for extension in split(extensions, '\t')
        let sep_pos = stridx(extension, ':')
        if sep_pos < 1
            if has_key(tag, 'kind')
                throw 'CtagsParseErr'
            endif
            let tag['kind'] = extension
        else
            let tag[extension[:sep_pos - 1]] = extension[sep_pos + 1:]
        endif
    endfor

    return tag
endfunction

" Find all classes derived from a given class, or a regex (preceded by a '/')
" The results are placed in the current windows location list.
function! Inherits(cls_or_regex)
    if a:cls_or_regex[0] == '/'
        let regex = a:cls_or_regex[1:]
    else
        let regex = '\<'.a:cls_or_regex.'\>$'
    endif
    let loc_list = []
    let tfiles = tagfiles()
    let tag_count = 0
    let found_count = 0
    for file in tfiles
        let tag_dir = fnamemodify(file, ':p:h')
        try
            for line in readfile(file)
                let tag_count += 1
                if tag_count % 10000 == 0
                    echo tag_count 'tags scanned,' found_count 'matching classes found. Still searching...'
                    redraw
                endif
                if line[0] == '!'
                    continue
                endif

                let tag = ParseCtagsRec(line, tag_dir)

                if has_key(tag, 'inherits')
                    let baselist = split(tag['inherits'], ',\s*')
                    for base in baselist
                        if match(base, regex) != -1
                            let location = {}
                            let location['filename'] = tag['filename']

                            let cmd = tag['cmd']
                            if cmd[0] == '/' || cmd[0] == '?'
                                let location['pattern'] = cmd[1:-2]
                            else
                                let location['lnum'] = str2nr(cmd)
                            endif

                            call add(loc_list, location)
                            let found_count += 1
                        endif
                    endfor
                endif
            endfor
        catch /^OptionErr$/
            echo 'Parsing error: Failed to parse an option.'
            return
        catch /^CtagsParseErr$/
            echo 'Parsing error: Tags files does not appear to be an Exuberant Ctags file.'
            return
        catch
            echo 'Could not read tag file:' file
            return
        endtry
    endfor
    call setloclist(0, loc_list)
    echo tag_count 'tags scanned,' found_count 'matching classes found.'
endfunction

command! -nargs=1 -complete=tag Inherits call Inherits('<args>')