Creating a prepopulated Windows Runtime Vector from C++/WinRT without going through an explicit std::vector

Raymond Chen

Last time, we saw that if you have to create a Windows Runtime Vector from C++/WinRT, do it as late as possible. And the extreme case of doing it as late as possible is creating it in the conversion function itself!

We’ll start with the na├»ve slow way:

namespace winrt
{
    using namespace winrt::Windows::Foundation;
}

winrt::IVector<winrt::Uri> GetUris()
{
    auto v = winrt::multi_threaded_vector<winrt::Uri>();
    v.Append(winrt::Uri(L"https://microsoft.com/"));
    v.Append(winrt::Uri(L"https://contoso.com/"));
    v.Append(winrt::Uri(L"https://fabrikam.com/"));
    return v;
}

This follows the inefficient pattern we discussed yesterday, where we create an empty IVector and then use the IVector methods to append elements to it.

We saw last time that we could do better by working with std::vector first, and converting it to an IVector as a final step:

winrt::IVector<winrt::Uri> GetOptions()
{
    std::vector<winrt::Uri> v;
    v.push_back(winrt::Uri(L"https://microsoft.com/"));
    v.push_back(winrt::Uri(L"https://contoso.com/"));
    v.push_back(winrt::Uri(L"https://fabrikam.com/"));
    return winrt::multi_threaded_vector(std::move(v));
}

And now that you have a std::vector, you can take advantage of vector-specific features like emplace_back, which treats its arguments as parameters which will be used to construct a winrt::Uri object in place in the vector.

winrt::IVector<winrt::Uri> GetUris()
{
    std::vector<winrt::Uri> v;
    v.emplace_back(L"https://microsoft.com/");
    v.emplace_back(L"https://contoso.com/");  
    v.emplace_back(L"https://fabrikam.com/"); 
    return winrt::multi_threaded_vector(std::move(v));
}

But there’s a way to do this without ever having to create a std::vector variable at all: You can create the vector on the fly as a parameter!

winrt::IVector<winrt::hstring> GetUris()
{
    // Alternative 1
    return winrt::multi_threaded_vector<winrt::Uri>({
        winrt::Uri(L"https://microsoft.com/"),
        winrt::Uri(L"https://contoso.com/"),
        winrt::Uri(L"https://fabrikam.com/"),
    });

    // Alternative 2
    return winrt::multi_threaded_vector(std::vector{
        winrt::Uri(L"https://microsoft.com/"),
        winrt::Uri(L"https://contoso.com/"),
        winrt::Uri(L"https://fabrikam.com/"),
    });
}

You can’t say

    return winrt::multi_threaded_vector({
        winrt::Uri(L"https://microsoft.com/"),
        winrt::Uri(L"https://contoso.com/"),
        winrt::Uri(L"https://fabrikam.com/"),
    });

because multi_threaded_vector cannot deduce the element type from the initializer list. (Maybe you can teach it and submit a pull request?)

But what you can do is take advantage of the fact that if you do specify the element type, then multi_threaded_vector will know that it needs a std::vector<winrt::Uri>, and then compiler will see from the vector constructors that the braced list is an initializer_list<winrt::Uri>, and then the compiler will see that it knows how to construct a winrt::Uri from a string literal, so it will automatically use the constructor to populate the initializer list. All this means that you don’t have to say winrt::Uri around each string:

winrt::IVector<winrt::hstring> GetUris()
{
    return winrt::multi_threaded_vector<winrt::Uri>({
        L"https://microsoft.com/",
        L"https://contoso.com/",
        L"https://fabrikam.com/",
    });
}

This is still a little frustrating because it does construct a bunch of temporary winrt::Uris to go into the initializer list, which are then copied into the vector. Instead, you can use the two-iterator version and let the vector do the constructing in-place.

winrt::IVector<winrt::hstring> GetUris()
{
    static constexpr PCWSTR uris[] = {
        L"https://microsoft.com/",
        L"https://contoso.com/",
        L"https://fabrikam.com/",
    };
    return winrt::multi_threaded_vector<winrt::Uri>(
        { std::begin(uris), std::end(uris) });
}

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