In Vim how to switch quickly between .h and .cpp files with the same name?

According to the Vim wiki there are quite a few suggested ways.

I will outline a few options from the article:

• a.vim or FSwitch.vim plugins
• using ctags
• :e %<.c or :e %<.h. %< represents the current file w/o the extension
• A quick mapping nnoremap <F4> :e %:p:s,.h$,.X123X,:s,.cpp$,.h,:s,.X123X$,.cpp,<CR>. Add this to your ~/.vimrc.

You can use the :r (root) filename modifier which removes the last extension (check out :h filename-modifiers for more information)

:e %:r.cpp

where

• % is shorthand for current filename.
• :r removes the extension
• .cpp simply appends that string at the end.

This effectively substitutes the current file's extension with another, then open the file with the newer extension.

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An even shorter way (courtesy of Peter Rincker),

:e %<.cpp

Relevant documentation at :h extension-removal