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Standard C++ data types and C++/WinRT

  • Cikk

With C++/WinRT, you can call Windows Runtime APIs using Standard C++ data types, including some C++ Standard Library data types. You can pass standard strings to APIs (see String handling in C++/WinRT), and you can pass initializer lists and standard containers to APIs that expect a semantically equivalent collection.

Also see Passing parameters into the ABI boundary.

Standard initializer lists

An initializer list (std::initializer_list) is a C++ Standard Library construct. You can use initializer lists when you call certain Windows Runtime constructors and methods. For example, you can call DataWriter::WriteBytes with one.

#include <winrt/Windows.Storage.Streams.h>

using namespace winrt::Windows::Storage::Streams;

int main()
{
    winrt::init_apartment();

    InMemoryRandomAccessStream stream;
    DataWriter dataWriter{stream};
    dataWriter.WriteBytes({ 99, 98, 97 }); // the initializer list is converted to a winrt::array_view before being passed to WriteBytes.
}

There are two pieces involved in making this work. First, the DataWriter::WriteBytes method takes a parameter of type winrt::array_view.

void WriteBytes(winrt::array_view<uint8_t const> value) const

winrt::array_view is a custom C++/WinRT type that safely represents a contiguous series of values (it is defined in the C++/WinRT base library, which is %WindowsSdkDir%Include\<WindowsTargetPlatformVersion>\cppwinrt\winrt\base.h).

Second, winrt::array_view has an initializer-list constructor.

template <typename T> winrt::array_view(std::initializer_list<T> value) noexcept

In many cases, you can choose whether or not to be aware of winrt::array_view in your programming. If you choose not to be aware of it then you won't have any code to change if and when an equivalent type appears in the C++ Standard Library.

You can pass an initializer list to a Windows Runtime API that expects a collection parameter. Take StorageItemContentProperties::RetrievePropertiesAsync for example.

IAsyncOperation<IMap<winrt::hstring, IInspectable>> StorageItemContentProperties::RetrievePropertiesAsync(IIterable<winrt::hstring> propertiesToRetrieve) const;

You can call that API with an initializer list like this.

IAsyncAction retrieve_properties_async(StorageFile const storageFile)
{
    auto properties{ co_await storageFile.Properties().RetrievePropertiesAsync({ L"System.ItemUrl" }) };
}

Two factors are at work here. First, the callee constructs a std::vector from the initializer list (this callee is asynchronous, so it's able to own that object, which it must). Second, C++/WinRT transparently (and without introducing copies) binds std::vector as a Windows Runtime collection parameter.

Standard arrays and vectors

winrt::array_view also has conversion constructors from std::vector and std::array.

template <typename C, size_type N> winrt::array_view(std::array<C, N>& value) noexcept
template <typename C> winrt::array_view(std::vector<C>& vectorValue) noexcept

So, you could instead call DataWriter::WriteBytes with a std::vector.

std::vector<byte> theVector{ 99, 98, 97 };
dataWriter.WriteBytes(theVector); // theVector is converted to a winrt::array_view before being passed to WriteBytes.

Or with a std::array.

std::array<byte, 3> theArray{ 99, 98, 97 };
dataWriter.WriteBytes(theArray); // theArray is converted to a winrt::array_view before being passed to WriteBytes.

C++/WinRT binds std::vector as a Windows Runtime collection parameter. So, you can pass a std::vector<winrt::hstring>, and it will be converted to the appropriate Windows Runtime collection of winrt::hstring. There's an extra detail to bear in mind if the callee is asynchronous. Due to the implementation details of that case, you'll need to provide an rvalue, so you must provide a copy or a move of the vector. In the code example below, we move ownership of the vector to the object of the parameter type accepted by the async callee (and then we're careful not to access vecH again after moving it). If you want to know more about rvalues, see Value categories, and references to them.

IAsyncAction retrieve_properties_async(StorageFile const storageFile, std::vector<winrt::hstring> vecH)
{
	auto properties{ co_await storageFile.Properties().RetrievePropertiesAsync(std::move(vecH)) };
}

But you can't pass a std::vector<std::wstring> where a Windows Runtime collection is expected. This is because, having converted to the appropriate Windows Runtime collection of std::wstring, the C++ language won't then coerce that collection's type parameter(s). Consequently, the following code example won't compile (and the solution is to pass a std::vector<winrt::hstring> instead, as shown above).

IAsyncAction retrieve_properties_async(StorageFile const storageFile, std::vector<std::wstring> vecW)
{
    auto properties{ co_await storageFile.Properties().RetrievePropertiesAsync(std::move(vecW)) }; // error! Can't convert from vector of wstring to async_iterable of hstring.
}

Raw arrays, and pointer ranges

Bearing in mind the caveat that an equivalent type may exist in the future in the C++ Standard Library, you can also work directly with winrt::array_view if you choose to, or need to.

winrt::array_view has conversion constructors from a raw array, and from a range of T* (pointers to the element type).

using namespace winrt;
...
byte theRawArray[]{ 99, 98, 97 };
array_view<byte const> fromRawArray{ theRawArray };
dataWriter.WriteBytes(fromRawArray); // the winrt::array_view is passed to WriteBytes.

array_view<byte const> fromRange{ theArray.data(), theArray.data() + 2 }; // just the first two elements.
dataWriter.WriteBytes(fromRange); // the winrt::array_view is passed to WriteBytes.

winrt::array_view functions and operators

A host of constructors, operators, functions, and iterators are implemented for winrt::array_view. A winrt::array_view is a range, so you can use it with range-based for, or with std::for_each.

For more examples and info, see the winrt::array_view API reference topic.

IVector<T> and standard iteration constructs

SyndicationFeed.Items is an example of a Windows Runtime API that returns a collection of type IVector<T> (projected into C++/WinRT as winrt::Windows::Foundation::Collections::IVector<T>). You can use this type with standard iteration constructs, such as range-based for.

// main.cpp
#include "pch.h"
#include <winrt/Windows.Web.Syndication.h>
#include <iostream>

using namespace winrt;
using namespace Windows::Web::Syndication;

void PrintFeed(SyndicationFeed const& syndicationFeed)
{
    for (SyndicationItem const& syndicationItem : syndicationFeed.Items())
    {
        std::wcout << syndicationItem.Title().Text().c_str() << std::endl;
    }
}

C++ coroutines with asynchronous Windows Runtime APIs

You can continue to use the Parallel Patterns Library (PPL) when calling asynchronous Windows Runtime APIs. However, in many cases, C++ coroutines provide an efficient and more easily-coded idiom for interacting with asynchronous objects. For more info, and code examples, see Concurrency and asynchronous operations with C++/WinRT.

Important APIs