Discuss on Groups View on GitHub


Cap'n Proto 0.5: Generics, Visual C++, Java, C#, Sandstorm.io

kentonv on 15 Dec 2014

Today we’re releasing Cap’n Proto 0.5. We’ve added lots of goodies!

Finally: Visual Studio

Microsoft Visual Studio 2015 (currently in “preview”) finally supports enough C++11 to get Cap’n Proto working, and we’ve duly added official support for it!

Not all features are supported yet. The core serialization functionality sufficient for 90% of users is available, but reflection and RPC APIs are not. We will turn on these APIs as soon as Visual C++ is ready (the main blocker is incomplete constexpr support).

As part of this, we now support CMake as a build system, and it can be used on Unix as well.

In related news, for Windows users not interested in C++ but who need the Cap’n Proto tools for other languages, we now provide precompiled Windows binaries. See the installation page.

I’d like to thank Bryan Boreham, Joshua Warner, and Phillip Quinn for their help in getting this working.

C#, Java

While not strictly part of this release, our two biggest missing languages recently gained support for Cap’n Proto:


Cap’n Proto now supports generics, in the sense of Java generics or C++ templates. While working on Sandstorm.io we frequently found that we wanted this, and it turned out to be easy to support.

This is a feature which Protocol Buffers does not support and likely never will. Cap’n Proto has a much easier time supporting exotic language features because the generated code is so simple. In C++, nearly all Cap’n Proto generated code is inline accessor methods, which can easily become templates. Protocol Buffers, in contrast, has generated parse and serialize functions and a host of other auxiliary stuff, which is too complex to inline and thus would need to be adapted to generics without using C++ templates. This would get ugly fast.

Generics are not yet supported by all Cap’n Proto language implementations, but where they are not supported, things degrade gracefully: all type parameters simply become AnyPointer. You can still use generics in your schemas as documentation. Meanwhile, at least our C++, Java, and Python implementations have already been updated to support generics, and other implementations that wrap the C++ reflection API are likely to work too.


0.5 introduces a (backwards-compatible) change in the way struct lists should be encoded, in order to support canonicalization. We believe this will make Cap’n Proto more appropriate for use in cryptographic protocols. If you’ve implemented Cap’n Proto in another language, please update your code!

Sandstorm and Capability Systems

Sandstorm.io is Cap’n Proto’s parent project: a platform for personal servers that is radically easier and more secure.

Cap’n Proto RPC is the underlying communications layer powering Sandstorm. Sandstorm is a capability system: applications can send each other object references and address messages to those objects. Messages can themselves contain new object references, and the recipient implicitly gains permission to use any object reference they receive. Essentially, Sandstorm allows the interfaces between two apps, or between and app and the platform, to be designed using the same vocabulary as interfaces between objects or libraries in an object-oriented programming language (but without the mistakes of CORBA or DCOM). Cap’n Proto RPC is at the core of this.

This has powerful implications: Consider the case of service discovery. On Sandstorm, all applications start out isolated from each other in secure containers. However, applications can (or, will be able to) publish Cap’n Proto object references to the system representing APIs they support. Then, another app can make a request to the system, saying “I need an object that implements interface Foo”. At this point, the system can display a picker UI to the user, presenting all objects the user owns that satisfy the requirement. However, the requesting app only ever receives a reference to the object the user chooses; all others remain hidden. Thus, security becomes “automatic”. The user does not have to edit an ACL on the providing app, nor copy around credentials, nor even answer any security question at all; it all derives automatically and naturally from the user’s choices. We call this interface “The Powerbox”.

Moreover, because Sandstorm is fully aware of the object references held by every app, it will be able to display a visualization of these connections, allowing a user to quickly see which of their apps have access to each other and even revoke connections that are no longer desired with a mouse click.

Cap’n Proto 0.5 introduces primitives to support “persistent” capabilities – that is, the ability to “save” an object reference to disk and then restore it later, on a different connection. Obviously, the features described above totally depend on this feature.

The next release of Cap’n Proto is likely to include another feature essential for Sandstorm: the ability to pass capabilities from machine to machine and have Cap’n Proto automatically form direct connections when you do. This allows servers running on different machines to interact with each other in a completely object-oriented way. Instead of passing around URLs (which necessitate a global namespace, lifetime management, firewall traversal, and all sorts of other obstacles), you can pass around capabilities and not worry about it. This will be central to Sandstorm’s strategies for federation and cluster management.

Other notes