Matt Kline Matt Kline - 1 year ago 83
C++ Question

Passing shared_ptr<Derived> as shared_ptr<Base>

What is the best method to go about passing a

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

I generally pass
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>();

I could use

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

but this seems sub-optimal for two reasons:

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

  • As I understand it,
    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
    by value, e.g.
    int foo(unique_ptr<bar> b)
    . Callers should
    the value into the function.

  • Functions that extend the lifetime of an object should take a
    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 Source

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();

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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