Class refers not to lifestyle but to position in the division of labor. Capitalism creates two (and only two) classes: the owners and the workers, or the bourgeoisie and the proletariat. Accordingly, there is no such thing as a middle class. This is a mistranslation of bourgeoisie, which describes the owing class in its moment of emergence, when it was “between” the nobility and the serfs. The French Revolution was a bourgeois revolution that mobilized elements of the working class. You will recall that Louis XVI was executed via the results of a vote in which nobles participated.
What is misnamed today the “middle class” is more properly described as a class-fraction whose interests are pulled in the direction of each of the two classes. This cadre or managerial class ranges from the so-called free professions (such as professors) to low level white collar workers and artists. Hegemony includes, among other things, a perpetual struggle to determine the class identifications of this class fraction. I argue that, because it does not own the means of production and increasingly finds its labor under the direction of others, it is part of the working class.
Well meaning academics from working class backgrounds have sometimes engaged in a form of self-flagellation whereby they insist that their privileged lifestyle is de facto evidence of their belonging to the owning class. Perhaps the stupidest rejoinder I ever read to my position was a statement to the effect that working class people don’t have to worry about taking a shower before going to work. So I guess wait staff are now members of the owning class. (By the way, I have in fact worked as a server, at a factory, playing piano for ballet classes, and in wholesale gifts, for example.)