Should member functions be "const" if they affect logical state, but not bitwise state?

Question

I'm writing a class that wraps a legacy C API that controls a hardware device. In a simplified example, I might have something like:

class device
{
public:
    void set_request(int data) { legacy_set_req(p_device, data); }
    int get_response() const   { return legacy_get_rsp(p_device); }
private:
    device_handle_t *const p_device;
};

The class itself has no bitwise state; therefore, I could choose to declare set_request() as const, and the compiler would be happy with that. However, from a semantic point-of-view, would this be the correct approach, given that it affects the observable behaviour of the object? (i.e. the encapsulated hardware device very much does have state.)

Answer 1

I believe that const should reflect logical const-ness, regardless of the internal representation. Just because your object contains only a pointer to something that changes, doesn't mean all your member functions should be const.

C++ even has the mutable concept for internal representation that needs to change even if conceptually the object does not. The const keyword is clearly not intended to represent "bitwise" const-ness.