VIM help in understanding X123X

Question

I came across very helpful mapping for VIM in vim Wiki

The mapping says:

map <F4> :e %:p:s,.h$,.X123X,:s,.cpp$,.h,:s,.X123X$,.cpp,<CR>

I wanted to expand it - For example cc/h files

But I couldn't understand the script. Mainly the X123X

Following pattern, I just did this:

map <F4> :e %:p:s,.h$,.X123X,:s,.cpp$,.h,:s,.X123X$,.cpp,:s,.cc$,.h,:s,.X123X$,.cc,<CR>

Now, it maps .CC->.H but not from .H->.CC instead it maps .H->.CPP.

Can any one help?

Answer 1

This mapping uses the special :s filename modifier (:help filename-modifiers) to swap the file extensions.

Because all substitutions are executed sequentially, the .h extension is first transformed into the .X123X placeholder value, so that the following .cpp → .h transformation doesn't apply to it.

This trick works well for a single pair; if you want to add alternatives, I would rather switch to a Vim function that does the swapping.

function! SwappedExtension()
    let [rest, ext] = [expand('%:r'), expand('%:e')]
    if ext ==? 'h'
        let ext = 'cpp'
    elseif ext ==? 'cpp'
        let ext = 'h'
    endif
    return rest . '.' . ext
endfunction
:nnoremap <F4> :e <C-r><C-r>SwappedExtension()<CR><CR>

Also, there are plugins that do this, e.g. the a.vim - Alternate Files quickly (.c --> .h etc).