Question

How can I use the members of enum state_t in the method bar::install without redefining it in the public class section of foo? Class member _state must stay protected.

Is it a good idea just to use another enum with similiar names for the members?

class foo
{
protected:
    int         _bla
    volatile enum state_t { _REC, _LIN, _OFF } _state;
public
    volatile enum { REC, LIN, OFF };
    state_t get_state(void) { return _state };
};

Within a method of another class:

void bar::install(foo *ptr)
{
    switch(ptr->get_state()) {
        case foo::REC: break;
        case foo::LIN: break;
        case foo::OFF: break;
    }
}

Answer 1

No, when you duplicate your enums you suffer extra effort when you modify them, you will also run into errors/warnings related to theirs types. I would make state_t public and keep _state protected by separating their definition, like this:

class foo
{   
public:
    enum state_t { REC, LIN, OFF };
protected:
    int         _bla
    volatile state_t _state;
public:
    state_t get_state(void) { return _state };
};