Whats the difference between decltype(type) and decltype(vector_of_type)?

Question

I've got a problem with decltype which doesn't work for vector.

Vector Eq is the same type as holder and itemGrabbed:

item* itemGrabbed;
item* holder;
std::vector<std::vector<item*>> Eq;

Everything is initialized, etc.

Eq stores pointers to "inherited" objects - for example, objects from class itemAxe.

Every item class and inherited objects have the same constructor with this argument -> grapicsData:

this->itemGrabbed = new std::remove_pointer<decltype(Eq[x][y]) > ::type{graphicsData};
this->itemGrabbed = new std::remove_pointer<decltype(holder) > ::type{graphicsData};

I guess I could just go with something like this:

item* temp=Eq[x][y];
this->itemGrabbed = new std::remove_pointer<decltype(temp) > ::type{graphicsData};

But I was wondering if it is possible to do it without temp?

The problem is that the 2nd option works but I need to use the 1st one. All that because I need different assign and copy operators.

How can I achieve this?

Answer 1

decltype is resolved by the compiler, at compilation time.

It does not (and cannot) take into account if a pointer to a base class refers to an instance of a derived class during runtime.

In your case:

holder and temp are both of type item*, and so the compiler will resolve these 2 decltypes usages in your question to item*. After applying std::remove_pointer they will become item (and you'll be able to new them).

As for Eq[x][y] - it is actually if type item*& (a reference to item*) and will stay the same after applying std::remove_pointer. Since you cannot use new with a refernce type, you will get a compilation error.
MSVC for example issues this error for it:

error C2464: '_Ty': cannot use 'new' to allocate a reference
     with
     [
            _Ty=item *&
     ]

Regarding type inference in runtime (asked about in comments):

C++ does not support full reflection, and so you cannot inspect the type of an object in runtime and use this type as is to create a new instance.
You will need to implement some code to do this heavy lifting, e.g. some kind of factory.
You can start with this post: Possible to instantiate object given its type in C++?.