Casting a container of shared_ptr

Question

Hi all,

I have a method

void foo(list<shared_ptr<Base>>& myList); 

Which I'm trying to call with a two different types of lists, one of DerivedClass1 and one of DerivedClass2

list<shared_ptr<DerivedClass1>> myList1; 
foo(myList1);
list<shared_ptr<DerivedClass2>> myList2; 
foo(myList2);

However this obviously generates a compiler error

error: a reference of type "std::list<boost::shared_ptr<Base>, std::allocator<boost::shared_ptr<Base>>> &" (not const-qualified) cannot be initialized with a value of type "std::list<boost::shared_ptr<DerivedClass1>, std::allocator<boost::shared_ptr<DerivedClass1>>>"

Is there any easy way to cast a container of shared_ptr? Of alternate containers that can accomplish this?

Update: Thanks to everyone who responded. Working within the confines of the language, it seems the best way to go while keeping the method 'as-is' is to use a container of shared_ptr and pass exactly that in (creating a new list at the call site).

I guess I pretty much already knew this, but I remembered reading about other parts of the boost library dealing with containers of shared_ptr and thought maybe it was solved more elegantly by someone else already. From my own further research however these seem to be geared more towards reducing overhead of shared_ptr in cases where a number of pointers are owned exclusively (therefore requiring a single lock per container rather than one per object in the container).

Thanks again, you guys are all awesome!

Answer 1

Note that list<shared_ptr<Base>> and list<shared_ptr<DerivedClass1>> (or even shared_ptr<Base> and shared_ptr<DerivedClass1>) are completely different types, even if DerivedClass1 inherits from Base. So casting one to the other would in fact be wrong.

Consider the case when foo attempts to add new elements of type Base to myList. If you could cast myList1 into a list<shared_ptr<Base>> (e.g. with an old-style C cast), and passed it to foo, the result would not be nice.

So the only proper solution I see is to create a new list<shared_ptr<Base>> object and copy the contents of the other list into it, then pass that to foo, as is demonstrated by @James McNellis.

Answer 2

You can't cast a container of one type to a container of another type. There are a few ways to create a new container from an existing container, if the type of object stored by the existing container is convertible to the type of object stored by the new container:

You can use std::copy to do the conversion element-by-element:

list<shared_ptr<Base> > baseList;
list<shared_ptr<Derived> > derivedList;
std::copy(derivedList.begin(), derivedList.end(), std::back_inserter(baseList));

You can also directly construct baseList using the begin and end iterators from derivedList:

list<shared_ptr<Base> > baseList(derivedList.begin(), derivedList.end());

If your function can take a const reference instead, you can construct a temporary in the function call:

typedef list<shared_ptr<Base> > BaseList;
foo(BaseList(derivedList.begin(), derivedList.end()));

Answer 3

The classical explanation why that wouldn't be a good idea goes as follows:

If you have a list<Apple*> nicely filled with Apples, and you were able to pass it to a function accepting a list<Fruit*>, then there is no stopping the function from putting new Oranges in the list. A list<Fruit*> is simply not substitutable for a list<Apple*> and there exists no relationship.

If you want a function that could work both with a list<Apple*> and list<Orange*>, you could use templates (whether with Iterators, or taking the Container itself, or taking a list<T*> - entirely your choice.

---

If you really badly wanted to pass a list<Apple*> as if a list<Fruit*>, then I suppose it would be technically possible to come up with proxies for the container / iterators, that use type erasure and what-not to cast the pointers internally / stop invalid casts (putting Oranges into list of Apples), so that you could make the function instead take a list_proxy<Fruit*> which accepts lists of derived types.

Something along the lines of (just a beginning):

#include <list>
#include <boost/shared_ptr.hpp>

template <class T>
class ListWrapperBaseAux
{
public:
    virtual ~ListWrapperBaseAux() {}
    virtual T* front() = 0;
};

template <class T, class U>
class ListWrapperAux: public ListWrapperBaseAux<T>
{
    std::list<U*>* list_ref;
public:
    ListWrapperAux(std::list<U*>* list_ref): list_ref(list_ref) {}
    virtual T* front() { return dynamic_cast<T*>(list_ref->front()); }
};

template <class T>
class ListProxy
{
    boost::shared_ptr<ListWrapperBaseAux<T> > list_ref;
public:
    template <class U>
    ListProxy(std::list<U*>& li): list_ref(new ListWrapperAux<T, U>(&li)) {}
    T* front() { return list_ref->front(); }
};

struct Fruit
{
    virtual ~Fruit() {}
};

struct Apple: Fruit {};
struct Orange: Fruit {};

void bake(ListProxy<Fruit> li)
{
    Fruit* f = li.front();
}

int main()
{
    std::list<Apple*> apples;
    bake(apples);

    std::list<Orange*> oranges;
    bake(oranges);
}