Try to use const as many as possible. Using const to a variable or a pointer means that the value or address can not be changed.
Using const to a func means that this function does not change any values inside the class.
The advantage of using const is that the compiler will tell you if you make some related mistakes.
Examples:
void A::func( const B * const b ); //in case b can not be changed in this function.
first const means only const functions of B can be used here
second const means the address of b can not be changed
double A::getValue() const { return value; } //no value change inside a class
