In C#, you can use the HttpUtility.ParseQueryString method to parse URL query strings. For C++/WinRT, you can use the Windows Runtime Windows.Foundation.WwwFormUrlDecoder class.

Uri uri{ L"https://www.youtube.com/watch?v=%64Qw4w9WgXcQ&t=43s" };
auto decoder = uri.QueryParsed();

// or you can parse a query string directly
WwwFormUrlDecoder decoder(L"v=%64Qw4w9WgXcQ&t=43s");

Once it’s parsed, you can pull out the pieces.

auto id = decoder.GetFirstValueByName(L"v");

The Www­Form­Url­Decoder is a bit difficult to work with, however, because the GetFirstValueByName method throws an exception if the key is not present at all. Since you are usually parsing query strings from potentially-untrusted sources, you can’t be certain that the value you want is present.

What you can do is convert the parsed query into some other format that is easier to work with, say, a std::multimap:

auto to_multimap(
    winrt::Windows::Foundation::WwwFormUrlDecoder const& decoder)
{
    std::multimap<winrt::hstring, winrt::hstring> parsed;
    for (auto&& entry : decoder) {
        parsed.emplace(entry.Name(), entry.Value());
    }
    return parsed;
}

Now you can pull out the values from the query in a way you are more comfortable with. And you can even use the usual multimap methods to extract multiple-valued keys, if that’s something you care about.

auto parsed = to_multimap(uri.QueryParsed());

// Does a "v=" key exist?
if (!parsed.contains(L"v"))
{
    error();
}

// Process all the "v=" keys.
auto values = parsed.equal_range(L"v");
for (auto it = values.first; it != values.second; ++it)
{
    process_value(it->second);
}

// Process the first "v=" key, if any.
auto it = parsed.lower_bound(L"v");
if (it != parsed.end() && it->first == L"v") {
    process_value(it->second);
}