I am wondering what is actually the preferred way of naming variables: some_variable or somevariable. Looking at some libraries, I have seen both. What is more, some wide-spread style conventions like Google C++ Style Guide allow both.
What do you prefer and why? Is there a rule or good practice which tells when to use which? And does the same applies to argument names in functions/methods?
And is mixing those two conventions a good idea? If yes, when should the first naming conventions be used, and when the second one?