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Detect that std::shared_ptr is holding a raw array (and obtaining its size)

April 10, 2020 by

Questions › Detect that std::shared_ptr is holding a raw array (and obtaining its size)
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Garmaine asked 3 years ago

I'm working on (yet another) C++ serialization library that supports standard types such as containers. In particular I want to support smart pointers.

C++17 introduced support for std::shared_ptr holding raw arrays (it knows to invoke delete [] in such cases). I need to detect that the shared_ptr is holding a raw array so that I can serialize it accordingly:

template <typename T>
void serialize(Writer& writer, const std::shared_ptr<T> ptr)
{
    // Writer has overloaded operator()

    if (ptr)
    {
        if (holdsRawArray(ptr)) // How to implement this???
        {
            auto arrayWriter = writer.array(); // RAII
            auto size = rawArraySize(ptr); // How to get this???
            for (std::size_t i=0; i<size; ++i) 
                arrayWriter(ptr[i]);
        }
        else
            writer(*ptr);
    }
    else
        writer(null);
}

How do I determine that a C++17 smart pointer contains a raw array? This information is erased in the element_type member typedef (via std::remove_extent_t). I also can't find anything in the smart pointer API that would reveal the size of the raw array.

I thought about using the detector idiom on operator[] and operator*, but it appears implementations are not required to undefine them if T is or isn't a raw array.

Is what I'm attempting even possible? I'm hoping I missed something, or that there's some trick I could use.

I know I can force users to use std::shared_ptr<std::array<N,T>> or std::shared_ptr<std::vector<T>> instead, but I just want to check if it's even possible for me to support smart pointers holding raw arrays.

Are you looking for the answer?
Original Question and Possible Answers can be found on `http://stackoverflow.com`

Question Tags: arrays, c++, c++17, shared-ptr, smart-pointers

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