Matt Kline Matt Kline - 2 months ago 7
C++ Question

Passing shared_ptr<Derived> as shared_ptr<Base>

What is the best method to go about passing a

shared_ptr
of a derived type to a function that takes a
shared_ptr
of a base type?

I generally pass
shared_ptr
s by reference to avoid a needless copy:

int foo(const shared_ptr<bar>& ptr);


but this doesn't work if I try to do something like

int foo(const shared_ptr<Base>& ptr);

...

shared_ptr<Derived> bar = make_shared<Derived>();
foo(bar);


I could use

foo(dynamic_pointer_cast<Base, Derived>(bar));


but this seems sub-optimal for two reasons:


  • A
    dynamic_cast
    seems a bit excessive for a simple derived-to-base cast.

  • As I understand it,
    dynamic_pointer_cast
    creates a copy (albeit a temporary one) of the pointer to pass to the function.



Is there a better solution?

Update for posterity:



It turned out to be an issue of a missing header file. Also, what I was trying to do here is considered an antipattern. Generally,


  • Functions that don't impact an object's lifetime (i.e. the object remains valid for the duration of the function) should take a plain reference or pointer, e.g.
    int foo(bar& b)
    .

  • Functions that consume an object (i.e. are the final users of a given object) should take a
    unique_ptr
    by value, e.g.
    int foo(unique_ptr<bar> b)
    . Callers should
    std::move
    the value into the function.

  • Functions that extend the lifetime of an object should take a
    shared_ptr
    by value, e.g.
    int foo(shared_ptr<bar> b)
    . The usual advice to avoid circular references applies.



See Herb Sutter's Back to Basics talk for details.

Answer

Although Base and Derived are covariant and raw pointers to them will act accordingly, shared_ptr<Base> and shared_ptr<Derived> are not covariant. The dynamic_pointer_cast is the correct and simplest way to handle this problem.

(Edit: static_pointer_cast would be more appropriate because you're casting from derived to base, which is safe and doesn't require runtime checks. See comments below.)

However, if your foo() function doesn't wish to take part in extending the lifetime (or, rather, take part in the shared ownership of the object), then its best to accept a const Base& and dereference the shared_ptr when passing it to foo().

void foo(const Base& base);
[...]
shared_ptr<Derived> spDerived = getDerived();
foo(*spDerived);

As an aside, because shared_ptr types cannot be covariant, the rules of implicit conversions across covariant return types does not apply when returning types of shared_ptr<T>.

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