error: passing 'const …' as 'this' argument of '…' discards qualifiers

Question

error: passing \'const A\' as \'this\' argument of \'void A::hi()\' discards qualifiers [-fpermissive]

I don\'t understand why I\'m getting

Answer 1

Your hi method is not declared as const inside your A class. Hence, the compiler cannot guarantee that calling a.hi() will not change your constant reference to a, thus it raises an error.

You can read more about constant member functions here and correct usage of the const keyword here.