Using reference as class members for dependencies

Question

I am going back to C++ after spending some time in memory-managed languages, and I\'m suddently kinda lost as to what is the best way to implement dependency injection. (I a

Answer 1

Summary: If you need to store a reference, store a pointer as a private variable and access it through a method which dereferences it. You can stick a check that the pointer isn't null in the object's invariant.

In depth:

Firstly, storing references in classes makes it impossible to implement a sensible and legal copy constructor or assignment operator, so they should be avoided. It is usually a mistake to use one.

Secondly, the type of pointer/reference passed in to functions and constructors should indicate who has responsibility for freeing the object and how it should be freed:

• std::auto_ptr - the called function is responsible for freeing, and will do so automatically when it's done. If you need copy semantics, the interface must provide a clone method which should return an auto_ptr.

• std::shared_ptr - the called function is responsible for freeing, and will do so automatically when it's done and when all other references to the object are gone. If you need shallow copy semantics the compiler generated functions will be fine, if you need deep copying the interface must provide a clone method which should return a shared_ptr.

• A reference - the caller has responsibility. You don't care - the object may be stack allocated for all you know. In this case you should pass by reference but store by pointer. If you need shallow copy semantics the compiler generated functions will be fine, if you need deep copying you're in trouble.

• A raw pointer. Who knows? Could be allocated anywhere. Could be null. You might be responsible for freeing it, you might not.

• Any other smart pointer - it should manage the lifetime for you, but you'll need to look at the documentation to see what the requirements are for copying.

Note that the methods which give you responsibility for freeing the object don't break DI - freeing the object is simply a part of the contract you have with the interface (as you don't need to know anything about the concrete type to free it).