Use an interface as shared pointer parameter [duplicate]

Question

How can I pass a class derived from an interface to a function taking the interface as a parameter?

I have an interface and a class set up something like this.

class Interface { public: virtual ~Interface() {} virtual void DoStuff() = 0; }; class MyClass : public Interface { public: MyClass(); ~MyClass(); void DoStuff() override; }; void TakeAnInterface(std::shared_ptr<Interface> interface); int main() { auto myInterface = std::make_shared<MyClass>(); TakeAnInterface(myInterface); }

The compiler complains about No matching function call to TakeAnInterface(std::shared_ptr<MyClass>&). Why doesn't function TakeAnInterface recieve the Interface class instead of MyClass?

cast from MyClass to Interface is automatic, while there is no obvious cast from std::shared_ptr<MyClass> to std::shared_ptr<Interface> — jpo38
– jpo38, Commented Jan 7, 2016 at 9:58
This code should compile fine. Which compiler are you using? — Christian Hackl
– Christian Hackl, Commented Jan 7, 2016 at 10:09
And did you post your real code? The error message should only come if you take a reference. Please post real code, not something you made up while you typed the question... — Christian Hackl
– Christian Hackl, Commented Jan 7, 2016 at 10:15

Some programmer dude · Accepted Answer · 2016-01-07 09:59:33Z

6

Because myInterface is an instance of std::shared_ptr<MyClass> and not std::shared_ptr<Interface>, and the classes are not automatically convertible to each other.

You can't use std::make_shared, you have to be explicit:

auto myInterface = std::shared_ptr<Interface>(new MyClass);

answered Jan 7, 2016 at 9:59

Some programmer dude

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4 Comments

YSC Over a year ago

Why is it downvoted?

Anders Over a year ago

Yes, that's it! Thank you :)

Payne Over a year ago

shared_ptr that is created like this auto myInterface = std::shared_ptr<Interface>(new MyClass); is not the same as auto myInterface = std::make_shared<MyClass>();. First will create pointer to pointer (2 jumps) so it's slower.. Check my solution: auto myInterface = static_cast<std::shared_ptr<Interface>>(std::make_shared<MyClass>());.

Some programmer dude Over a year ago

@Payne That's an obfuscating and most likely premature optimization if I've ever seen one. Also, the "problem" with my solution isn't about pointer to pointer (double indirection), but rather that there are two memory allocations instead of just one.

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