NeedsTeXFormat{LaTeX2e}
ProvidesClass{myclass}
RequirePackage{kvoptions}

DeclareBoolOption{foo}

ProcessKeyvalOptions*

ifmyclass@foo
def@title{foo}
else
def@title{bar}
fi

ProcessOptionsrelax

LoadClass{article}

documentclass[foo]{myclass}

begin{document}

maketitle
end{document}

LaTeX Warning: Unused global option(s):
    [foo].

ifx@unusedoptionlist@emptyelse
    @latex@warning@no@line{Unused global option(s):^^J%
            @spaces[@unusedoptionlist]}%
  fi

NeedsTeXFormat{LaTeX2e}
ProvidesClass{testkeyval}[2018/01/19 Test key-value options]
RequirePackage{kvoptions}
DeclareStringOption[?]{foo}
DeclareDefaultOption{PassOptionsToClass{CurrentOptionKey}{article}}
ProcessKeyvalOptions*
newcommandthefoo{`testkeyval@foo'}
LoadClass{article}

documentclass[foo=bar]{testkeyval}
%documentclass[foo=bar,11pt]{testkeyval} % passes 11pt to article
%documentclass[11pt]{testkeyval} % passes 11pt to article
%documentclass{testkeyval} % default copyright and default fontsize for article
%documentclass[foo]{testkeyval} % error

begin{document}
% Number of 'x's on second line: 10pt 0, 11pt 3.
noindent
x x x x x x x x x x x x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x

foo is thefoo.
end{document}