How to clone a Windows Runtime vector in the face of possible concurrent modification, part 3

Raymond Chen

Last time, we cloned a Windows Runtime vector in the face of possible concurrent modification, but we ran into trouble with std::vector<bool>.

As I noted some time ago, the C++ language defines a specialization std::vector<bool> which represents a packed bit array, rather than defining a separate type like std::bitvector. This has made a lot of people very angry and has been widely regarded as a bad move.

In our case, the specialization of std::vector<bool> breaks our clone_as_vector function, since it needs a winrt::array_view<bool>, which needs a C-style array of bool objects, not a packed bit array.

We’ll have to detect the bool case in our function and substitute a std::unique_ptr<bool[]>.

template<typename V>
auto clone_as_vector(V const& v)
-> std::vector<decltype(v.GetAt(0))>
{
    using T = decltype(v.GetAt(0));
    std::conditional_t<         
        std::is_same_v<T, bool>,
        std::unique_ptr<bool[]>,
        std::vector<T>> temp;   
    uint32_t expected;
    uint32_t actual;
    do {
        expected = v.Size();
        if constexpr (std::is_same_v<T, bool>) {             
            temp = std::make_unique<bool[]>(expected + 1);   
            actual = v.GetMany(0,                            
                winrt::array_view(temp.get(), expected + 1));
        } else {                                             
            temp.resize(expected + 1, winrt_empty_value<T>());
            actual = v.GetMany(0, temp);
        }
    } while (actual > expected);
    if constexpr (std::is_same_v<T, bool>) {                
        return std::vector(temp.get(), temp.get() + actual);
    } else {                                                
        temp.erase(temp.begin() + actual, temp.end());
        return temp;
    }
}

If the value type of the vector is a bool, then the temp changes to a std::unique_ptr<bool[]>, and that means that we have to change how we resize the array (namely by replacing it with a new allocation) and how we generate the array_view<bool> (by using the pointer + size constructor).

After the loop has captured the elements, we convert our C-style array of bool into a std::vector<bool>.

Now that I looked at it some more, it seems that there is barely any shared code at all. May as well just make it two functions glued together.

template<typename V>
auto clone_as_vector(V const& v)
-> std::vector<decltype(v.GetAt(0))>
{
    using T = decltype(v.GetAt(0));
    uint32_t expected;
    uint32_t actual;
    if constexpr (std::is_same_v<T, bool>) {
        std::unique_ptr<bool[]> temp;
        do {
            expected = v.Size();
            temp = std::make_unique<bool[]>(expected + 1);
            actual = v.GetMany(0,
                winrt::array_view(temp.get(), expected + 1));
        } while (actual > expected);
        return std::vector(temp.get(), temp.get() + actual);
    } else {
        std::vector<T> temp;
        do {
            expected = v.Size();
            temp.resize(expected + 1, winrt_empty_value<T>());
            actual = v.GetMany(0, temp);
        } while (actual > expected);
        temp.erase(temp.begin() + actual, temp.end());
        return temp;
    }
}

Whew, we took care of the pesky std::vector<bool>.

Next time, we’ll look at the potential infinite loop and whether it offers a denial of service attack.

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