Boost shared_ptr passing a derived class

Question

So I have something like so

class baseclass {
....
}

class derived : public baseclass {
...
}

void func(boost::shared_ptr<baseclass>& a){
//stuff
}

boost::shared_ptr<derived> foo;

func(foo);

Will this work? I assume not because its not the same type, however I do not posses the ability to cast it to the right type, so is there any work around that you can think of that will make this work?

Edit: the reason I can't do the cast to my knowledge is because I'm doing a sort on a vector of type boost::shared_ptr<derived>so I only call sort with vec.begin() and vec.end()

score 4 · Accepted Answer · edited Aug 12 '15 at 12:40

4

shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*. In particular, shared_ptr<T> is implicitly convertible to shared_ptr<T const>, to shared_ptr<U> where U is an accessible base of T, and to shared_ptr<void>.

http://www.boost.org/doc/libs/1_53_0/libs/smart_ptr/shared_ptr.htm#Members

As your code is now, though, it won't work. The implicit constuction of the boost::shared_ptr<baseclass> object must be bound to a const reference, or be copied to persist beyond construction.

Consider either

void func(boost::shared_ptr<baseclass> a)

or

void func(const boost::shared_ptr<baseclass>& a)

edited Aug 12 '15 at 12:40

Tarc

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answered Jun 04 '13 at 16:09

Steven Maitlall

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Sorry I actually do have the const in my code I just didn't put it up here. – csteifel Jun 04 '13 at 16:15
With const& it works, however be aware that it involves temporary so bumping the refcount up/down, unlike with matching type. – Balog Pal Jun 04 '13 at 16:44

score 1 · Answer 2 · answered Jun 04 '13 at 16:10

1

Yes, it will work. Wouldn't it have been quicker just to compile it than to ask here?

shared_ptr has a templated converting constructor that allows implicit conversions from shared_ptr<D> to shared_ptr<B> if (and only if) D* is implicitly convertible to B*

answered Jun 04 '13 at 16:10

Jonathan Wakely

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but would a temporary created such way happily bind to & ? – Balog Pal Jun 04 '13 at 16:12
Does it do a dynamic cast under the covers to achieve this? So it will do type checking or is it a static cast? – csteifel Jun 04 '13 at 16:16
@BalogPal, oops, I missed the `&` although the comment on another answer indicates it's a `const&` so the temporary would work – Jonathan Wakely Jun 04 '13 at 16:39
@legion, it's an implicit conversion, you don't need type checking for a derived-to-base conversion, it's always valid! – Jonathan Wakely Jun 04 '13 at 16:40

score 1 · Answer 3 · answered Jun 04 '13 at 16:11

1

I'd be very much surprised. You need to pass lvalue of the proper type. For that to work the class should be base class and it is not the case.

NOTE: If if your function would take the ptr by value or const& other rules would apply.

answered Jun 04 '13 at 16:11

Balog Pal

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Boost shared_ptr passing a derived class

3 Answers3