No member function can be invoked on a const-qualified object unless the member function is declared "const".
Qualifying member functions that don’t mutate their object with the "const" qualifier makes your interface easier to understand; you can deduce
without diving into implementation if a member function is going to mutate its object.
Also, const-qualified member functions make working with const-qualified objects possible. The compiler ensures that only member functions that are
declared "const" can be invoked on "const" objects. Avoiding declaring non-mutating member functions const might break const-correctness: it will
not be possible to invoke such non-mutating functions on const-qualified objects.
Noncompliant code example
class A {
void f(){ // Noncompliant
std::cout<< "f doesn't mutate A";
}
};
Compliant solution
class A {
void f() const {
std::cout<< "f doesn't mutate A";
}
};
Exceptions
Virtual member functions that don’t mutate their objects don’t necessarily need to be declared const. This might be done in order to allow them to
be overridden by non-const functions.
