How to refer to a method defined in a derived class only, using an upcast unique_ptr pointer in C++?

Question

Assume following classes

class Base
{
   void Doajob(a,b){...}
}

class Derived: public Base
{
   void Doanotherjob(a,b,c){...}
}

I have defined a pointer as follows:

 auto ptr= unique_ptr<Base>(new Derived(name));

Now I want to access Doanotherjob using the ptr pointer:

ptr->Doanotherjob(a,b,c); // ERROR
((unique_ptr<Base>) ptr)->Doanotherjob(a,b,c); // ERROR
((unique_ptr<Derived>) ptr)->Doanotherjob(a,b,c); // ERROR

Is that even a right thing to do? What is the syntax?

On a side note, downcasting to a derived class violates LSP and is usually indicative of a flaw in your design. — Julian
– Julian, Commented Mar 30, 2015 at 21:00
Even with a regular pointer you wouldn't able to make this call, unless via a downcast, since you don't access an overridden function — vsoftco
– vsoftco, Commented Mar 30, 2015 at 21:04
Base must have a virtual destructor, otherwise the unique_ptr will cause UB when it tries to delete the pointer — M.M
– M.M, Commented Mar 31, 2015 at 21:01
I guess you meant to derive Derived from Base, rather than from itself — M.M
– M.M, Commented Mar 31, 2015 at 21:02

Chad · Accepted Answer · 2015-03-31 19:43:46Z

5

If you know for sure that the downcast is safe, you can use static_cast.

static_cast<derived*>(ptr.get())->DoAnotherJob(...);

However, if you made DoAnotherJob() virtual in base, then the downcast is unnecessary. This is a much more traditional object-oriented approach.

As stated in the comments below, dynamic_cast let's you do this cast and verify the result:

if(auto d = dynamic_cast<derived*>(ptr.get())
   d->DoAnotherJob();

edited Mar 31, 2015 at 19:43

answered Mar 30, 2015 at 20:44

Chad

19.2k4 gold badges51 silver badges65 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

vsoftco Over a year ago

perhaps dynamic_cast as it allows you to test whether the result is ok?

peyman Over a year ago

I could not make 'code' DoAnotherjob() virtual in base as type of c is not defined in Base. I tried making a virtual DoAnotherjob(TypeofA a,TypeofB b, vector<Derived::TypeofCelement> c) but it did not work. So I tried to bypass the Base class criteria and send the arguments directly to the derived class. Also I have a container of pointers to Base class which may or may not be possible to be downcasted to derive class. I think I can use dynamic_cast o check as above mentioned. Am I right?

Chad Over a year ago

Yes dynamic_cast in the example above will return nullptr if the conversion fails so the body of the if statement will not be executed in that case.

M.M Over a year ago

The dynamic_cast version will only work if Base has a virtual function. However using the unique_ptr is UB anyway if Base did not have a virtual destructor.

Collectives™ on Stack Overflow

How to refer to a method defined in a derived class only, using an upcast unique_ptr pointer in C++?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related