我如何安全地分享班级成员？

Question

I'm struggling to organise a program. I have a class Solver , which looks like this:

class Solver {
   ...
   VarOrder order;
   ...
   vector<Constraint> watches;
   vector<Constraint> undos;
   ...
};

Where of course Solver uses these members to its business. However, VarOrder also needs to use watches and undoes to do its business and both classes need to be able to see each other's modifications. So it seems like I need to pass a pointer to the watches and undoes member variables into the constructor of VarOrder . I guess I should do that with shared_ptr to be safe, but pointers to vectors seems like opening a massive can of awful.

I could also use the "friends" feature of C++, but that seems wrong as well, since VarOrder only requires access to two of Solver 's members, not all of them, which seems like it would be weird design.

Probably my class design is wrong, but short of creating global data, I don't know how to reorganize this. Any advice appreciated.

Thank you.

Answer 1

No need to use shared pointers in this case, as those vectors have been allocated on the stack along with VarOrder and so they will all exist for the lifetime of each other. Have VarOrder take a reference to the others (so no null pointer issues).

class VarOrder {
private:
    vector<Constraint>& _watches;
    vector<Constraint>& _undos;

public:
    VarOrder(vector<Constraint>& watches,
             vector<Constraint>& undos) : _watches(watches)
                                        , _undos(undos) {
    }; // eo ctor
}; // eo class VarOrder

class Solver {
private:
    vector<Constraint> _watches;
    vector<Constraint> _undos;
    VarOrder _order;

public:
    Solver() : _undos()
             , _watches()
             , _order(_watches, _undos) {
    }; // eo ctor
}; // eo class Solver

Note that members are initialised in the initialiser-list in the order in which they are declared in the class , so by the time _order comes around to construction, _undos and _watches are fully constructed.