Inside the C++/CX Design

Jim SpringfieldHello. This is Jim Springfield, an architect on the Visual C++ team.

Today, I want to give some insight into the new language extensions, officially called C++/CX, which was designed to support the new API model in Windows 8. If you attended //BUILD/, watched some of the sessions online, or have been playing with the prerelease of Visual Studio, you probably have seen some of the “new” syntax. For anyone who is familiar with C++/CLI (i.e. the language extensions we provide for targeting the CLR), the syntax shouldn’t seem much different.

Please note, however, that while the C++/CX syntax is very similar to C++/CLI, the underlying implementation is very different, it does not use the CLR or a garbage collector, and it generates completely native code (x64, x64, ARM depending on target).

Early on in the design of our support for Windows 8, we looked at many different ideas including a pure library approach as well as various ways to integrate support in the language. We have a long history of supporting COM in the Visual C++ team. From MFC to ATL to #import to attributed ATL. We also have a good bit of experience at targeting the CLR including the original managed extensions, C++/CLI, and the IJW support for compiling native code to MSIL. Our design team consisted of seven people and included people who worked on these and who have lots of experience in libraries, compiler implementation, and language design.

We actually did develop a new C++ template library for Windows 8 called WRL (Windows Runtime Library) that does support targeting Windows 8 without language extensions. WRL is quite good and it can be illuminating to take a look at it and see how all of the low-level details are implemented. It is used internally by many Windows teams, although it does suffer from many of same problems that ATL does in its support of classic COM.

  1. Authoring of components is still very difficult. You have to know a lot of the low-level rules about interfaces.
  2. You need a separate tool (MIDL) to author interfaces/types.
  3. There is no way to automatically map interfaces from low-level to a higher level (modern) form that throws exceptions and has real return values.
  4. There is no unification of authoring and consumption patterns.

With some of the new concepts in the Windows Runtime, these drawbacks become even more difficult than in classic COM/ATL. Interface inheritance isn’t vtable-based like it is in classic COM. Class inheritance is based on a mechanism similar to aggregation but with some differences including support for private and protected interfaces. We quickly realized that although there is a need for a low-level tool like WRL, for the vast majority of uses, it is just too hard to use and we could do a lot better while still preserving performance and providing a lot of control.

The #import feature that was available in VC6 provides a good mechanism for consuming COM objects that have a type library. We thought about providing something similar for the Windows Runtime (which uses a new .winmd file), but while that could provide a good consumption experience, it does nothing for authoring. Given that Windows is moving to a model where many things are asynchronous, authoring of callbacks is very important and there aren’t many consumption scenarios that wouldn’t include at least some authoring. Also, authoring is very important for writing UI applications as each page and user-defined control is a class derived from an existing Runtime class.

The design team spent a lot of time discussing what consumption of Windows Runtime components should look like. We decided early on that we should expose classes and interfaces at a higher level than what the ABI defines. Supporting modern C++ features such as exceptions was deemed to be important as well as mapping the Runtime definition of inheritance (both for interfaces and classes) to C++ in such a way that it was natural. It quickly became clear that we would need some new type categories to represent these as we couldn’t change what the existing C++ ABI meant. We went through a lot of different names and it wasn’t until we decided to use the ^ that we also decided to use ref class to indicate the authoring of a Windows Runtime class.

We also spent a lot of time exploring various approaches to how to hold onto a pointer to a WinRT class or interface. Part of this decision was also how to tell the difference between a low-level version of an interface and the high-level version of the interface. We had a lot of different proposals including just using a *, using * with a modifier, and using various other characters such as the ‘@’ symbol. In the original extensions we did for managed code, we in fact did use a * with a modifier (__gc). We realized we would have many of the same problems if we followed that route. Some of the breakthroughs came when we started thinking about what the type of a pointer dereference would be. This made us realize that what we were doing was similar to what we did when C++/CLI was designed. At one point, someone suggested “Why don’t we just use the ^ symbol?” After the laughter died down, it started making a lot of sense. On design point after design point, we often came to the same design decision we had made for C++/CLI.

Many of the concepts we were trying to express were already present in the C++/CLI syntax. Given that reference counting is a form of garbage collection, using ^ to represent a “refcounted” pointer in ZW fits quite well. Dereferencing of a ^ yields a %, also like C++/CLI. While many concepts are expressed the same way, there are a few areas where we decided to deviate from C++/CLI. For example, in C++/CX, the default interface on a class is specified through an attribute in the interface list while in C++/CLI it is an attribute on the class itself.

In C++/CX we have a much better story than C++/CLI when it comes to interoperating references types with regular types. In C++/CLI, managed objects can move around in memory as the garbage collector runs. This means you can’t get the real address of a member (without pinning) or even embed anything except primitive types (i.e. int) into your class. You also cannot put a ^ into a native class or struct. In C++/CX, objects do not move around in memory and thus all of these restrictions are gone. You can put any type into a ref class and you can put a ^ anywhere. This model is much friendlier to normal C++ types and gives the programmer more flexibility in C++/CX.

We will be providing more insight into our design over the coming months. If there are specific things you would like to know more about, please let us know.