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I’m writing a wrapper that, for the purpose of this question, does nothing but wraps a SequenceContainer ( http://en.cppreference.com/w/cpp/concept/SequenceContainer ) and reproduces all functionality of the SequenceContainer concept the wrapped container offers.

My attempt to write such a wrapper looks like this:

template<class Container>
class SequenceContainerWrapper
{
  Container cont;
public:
  using value_type = typename Container::value_type;
  using reference = typename Container::reference;
  using size_type = typename Container::size_type;
  SequenceContainerWrapper(initializer_list<value_type> init) : cont(init) {}
  reference operator[] (size_type n) {return cont[n];}
  // lots of other code that does nothing but wrapping
};

And yes – I can use it, for example, with std::vector. The below code compiles and works as expected ( http://ideone.com/sYeIeJ ):

int main()
{
    SequenceContainerWrapper<vector<int>> vec({1,2,3,4});
    cout << vec[2] << '\n';
}

However, this constructor won’t work with std::array. This snippet does not compile ( http://ideone.com/5nZhar ):

int main()
{
    SequenceContainerWrapper<array<int, 4>> arr({1,2,3,4});
    cout << arr[2] << '\n';
}

This is because ( http://en.cppreference.com/w/cpp/concept/SequenceContainer#cite_note-1 ):

std::array supports assignment from a braced-init-list, but not from an std::initializer_list

So how can I reproduce in my wrapper the possibility to initialize an std::array with a braced-init-list without sacrificing the genericness of my wrapper and introducing solutions that will cripple the wrapper’s compatibility with e.g. std::vector?

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1 Answer 1

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Instead of using:

SequenceContainerWrapper(initializer_list<value_type> init) : cont(init) {}

you could use:

SequenceContainerWrapper(Container init) : cont(std::move(init)) {}

See it working at http://ideone.com/MzRQGC.

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12 Comments

Thank you. But, outta simple curiosity: If a copy constructor is everything you need and will also cover initializer list initialization, then why does std::vector provide both constructors? en.cppreference.com/w/cpp/container/vector/vector We have both vector( const vector& other ); and vector( std::initializer_list<T> init ); there. Wouldn’t vector( const vector& other ); suffice?
My thinking is, that with your excellent solution, there will be one constructor overload of std::vector I will not explicitly wrap; so, assuming that the standard library doesn’t provide redundant functions, there will have to be at least one use case of std::vector I will fail to cover…
@gaazkam, if std::vector did not provide vector( std::initializer_list<T> init ), you won't be able to use a std::initializer_list to construct a temporary std::vector in the call to construct a SequenceContainerWrapper.
So… there still be one use case of std::vector I won’t cover: SequenceContainerWrapper<SequenceContainerWrapper<std::vector<int>>> vec2({1,2,3,4}); ideone.com/Kv5YtM This is unfortunately an important use-case, since I do nest these wrappers, each alternating the container’s behavior slightly, to achieve the desired outcome with the correct chain of slightly different wrappers.
With the SequenceContainerWrapper(initializer_list<value_type> init) : cont(init) {} I can nest these wrappers ideone.com/EKywvb , however unfortunately this ofc breaks std::array... Dang.
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