CODEWITHSUNDEEP
c++templatesc++11casting
user3072517
user3072517

Reputation: 533

Is there a way to simulate downcasting by reference

So, I have something along the lines of these structs:

struct Generic {}
struct Specific : Generic {}

At some point I have the the need to downcast, ie:

Specific s = (Specific) GetGenericData();

This is a problem because I get error messages stating that no user-defined cast was available.

I can change the code to be:

Specific s = (*(Specific *)&GetGenericData())

or using reinterpret_cast, it would be:

Specific s = *reinterpret_cast<Specific *>(&GetGenericData());

But, is there a way to make this cleaner? Perhaps using a macro or template?

I looked at this post C++ covariant templates, and I think it has some similarities, but not sure how to rewrite it for my case. I really don't want to define things as SmartPtr. I would rather keep things as the objects they are.

Upvotes: 2

Views: 948

Answers (4)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275800

I assume here that your data is simple.

struct Generic {
  int x=0;
  int y=0;
};
struct Specific:Generic{
  int z=0;
  explicit Specific(Generic const&o):Generic(o){}

  // boilerplate, some may not be needed, but good habit:
  Specific()=default;
  Specific(Specific const&)=default;
  Specific(Specific &&)=default;
  Specific& operator=(Specific const&)=default;
  Specific& operator=(Specific &&)=default;
};

and bob is your uncle. It is somewhat important that int z hae a default initializer, so we don't have to repeat it in the from-parent ctor.

I made thr ctor explicit so it will be called only explicitly, instead of by accident.

This is a suitable solution for simple data.

Upvotes: 1

user3072517
user3072517

Reputation: 533

Ok, after some additional study, I am wondering if what is wrong with doing this:

struct Generic {}
struct Specific : Generic {
    Specific( const Generic &obj ) : Generic(obj) {}
}

Correct me if I am wrong, but this works using the implicit copy constructors.

Assuming that is the case, I can avoid having to write one and does perform the casting automatically, and I can now write:

Specific s = GetGenericData();

Granted, for large objects, this is probably not a good idea, but for smaller ones, will this be a "correct" solution?

Upvotes: 0

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275800

So the first step is to realize you have a dynamic state problem. The nature of the state you store changes based off dynamic information.

struct GenericState { virtual ~GenericState() {} }; // data in here
struct Generic;
template<class D>
struct GenericBase {
  D& self() { return *static_cast<D&>(*this); }
  D const& self() const { return *static_cast<D&>(*this); }
  // code to interact with GenericState here via self().pImpl
  // if you have `virtual` behavior, have a non-virtual method forward to
  // a `virtual` method in GenericState.
};

struct Generic:GenericBase<Generic> {
  // ctors go here, creates a GenericState in the pImpl below, or whatever
  ~GenericState() {} // not virtual
private:
  friend struct GenericBase<Generic>;
  std::unique_ptr<GenericState> pImpl;
};
struct SpecificState : GenericState {
  // specific stuff in here, including possible virtual method overrides
};

struct Specific : GenericBase<Specific> {
  // different ctors, creates a SpecificState in a pImpl
  // upcast operators:
  operator Generic() && { /* move pImpl into return value */ }
  operator Generic() const& { /* copy pImpl into return value */ }
private:
  friend struct GenericBase<Specific>;
  std::unique_ptr<SpecificState> pImpl;
};

If you want the ability to copy, implement a virtual GenericState* clone() const method in GenericState, and in SpecificState override it covariantly.

What I have done here is regularized the type (or semiregularized if we don't support move). The Specific and Generic types are unrelated, but their back end implementation details (GenericState and SpecificState) are related.

Interface duplication is avoided mostly via CRTP and GenericBase.

Downcasting now can either involve a dynamic check or not. You go through the pImpl and cast it over. If done in an rvalue context, it moves -- if in an lvalue context, it copies.

You could use shared pointers instead of unique pointers if you prefer. That would permit non-copy non-move based casting.

Upvotes: 0

Barry
Barry

Reputation: 303527

It looks like GetGenericData() from your usage returns a Generic by-value, in which case a cast to Specific will be unsafe due to object slicing.

To do what you want to do, you should make it return a pointer or reference:

Generic* GetGenericData();
Generic& GetGenericDataRef();

And then you can perform a cast:

// safe, returns nullptr if it's not actually a Specific*
auto safe = dynamic_cast<Specific*>(GetGenericData());

// for references, this will throw std::bad_cast
// if you try the wrong type
auto& safe_ref = dynamic_cast<Specific&>(GetGenericDataRef());

// unsafe, undefined behavior if it's the wrong type,
// but faster if it is
auto unsafe = static_cast<Specific*>(GetGenericData());

Upvotes: 7

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