C++11 Composite Pattern with Smart Pointers

Question

I am working on a personal project to familiarize myself with C++11 and Boost.

I have a inheritance relationship with a UrlExtractor base class, with a TagUrlExtractor derived class and a CompoundExtractor derived class. A CompoundExtractor can contain any number of UrlExtractors , thus implementing the composite pattern .

I am curious what the method signature for CompositeExtractor.addExtractor should look like. Since I'm dealing with a polymorphic type, I am forced to work with pointers (or references). I want to use the new smart pointers, specifically shared_ptr .

I tried writing the addExtractor member function like this:

void addExtractor(shared_ptr<UrlExtractor> extractor);

However, I get a compile error if I try to write:

compound.addExtractor(new TagUrlExtractor());

The issue is that it would take two implicit conversions to create a shared_ptr<UrlExtractor> : one to the UrlExtractor base class and second to the shared_ptr .

I could call the function like this:

compound.addExtractor(shared_ptr<UrlExtractor>(new TagUrlExtractor));

However, this is very verbose. How is this situation handled normally?

Answer 1

The issue is that it would take two implicit conversions to create a shared_ptr: one to the UrlExtractor base class and second to the shared_ptr

That's not the issue: the issue is that the constructor of shared_ptr that accepts a raw pointer is marked as explicit , so it can't be used in a copy-initialization context (function parameters are copy-initialized, as specified by § 8.5/15).

You could (and actually, should) use std::make_shared :

compound.addExtractor(std::make_shared<TagUrlExtractor>());

std::make_shared() has the advantage of being exception-safe and performing one dynamic allocation less than when you are initializing a shared pointer from a new expression (also see this Q&A on StackOverflow ).