As offered by the comment you responded to: trailing underscore. And yes: since leading underscore is reserved, and something needed to be reserved for reasonably-good reasons, m_ is preferable to _ if some reason you simply refuse to use a trailing modifier.
> if some reason you simply refuse to use a trailing modifier
I'm not going to make a big deal about it, but it makes more sense to me to put the scope of a variable at the front. The front is where you put foo. and foo-> and foo[], after all.
Leading underscore is only reserved if the following letter is uppercase. And it's fairly common for C++ code to have coding style that mandates that fields start with a lowercase letter anyway.
