It's hard to think of what operator+ does for types other than numbers (mathematical addition) or strings (concatenation). I would therefore not use operator+ for your case. Instead create a member (or friend, or whatever) with a different name.
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class reg {
public:
reg operator+( int rhs ) const;
};
or
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// Note: Need friendship since you have to access private members of reg
reg operator+( const reg& lhs, int rhs )
{
}