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- Meta Programming Binary.
- A gentle introduction to Template Metaprogramming with C++. It's great to see template meta-programming really starting to be used in the real-world.
Maya's Programming & Electronics Blog. Recently I had to add a Websocket server to a C++ project. Some research showed that the options here aren’t too many. There are a few C- based options, and one can of course pick the Websocket module from the POCO libraries . Since this particular project is written in C++ I much preferred a purely C++ solution, preferably stand- alone. Ultimately I picked the (creatively named) Websocket++ library . Main arguments here were as mentioned being an object- oriented C++ solution, without significant dependencies, as well as ease of implementation thanks to it being a header- only library.
Websocket++ is a fairly modular library, making heavy use of templating to assemble various configurations, end points and similar into one coherent whole. As the basis for the transport module can for example pick from iostreams (very slow) and ASIO. For the latter one can pick between Boost ASIO and stand- alone ASIO. There is also the option of using no C++1. Boost alternatives instead. Since this project involved a number of compile targets, not all of which featured a C++1.
Boost dependency using its ASIO and system library, as well as various other header- only dependencies. After starting the actual integration of the library into my project, I did however find out that the quality of the documentation is very. The documentation is split between the Git. Hub site and the author’s own site, with most of this documentation being completely and utterly outdated.
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Only after significant amounts of trial and error did I manage to get a fully working implementation. To save others the trouble, I would like to hereby present a (simplified and altered) version of my implementation. I hope it will be useful. Let’s move on to the header file of our implementation.
This will allow us to use the Websocket functionality from multiple classes. Websocket++ is thread- safe, so all we have to worry about is multi- thread level access to our own data structures and variables. Moving on to the implementation, we can see first the usual static initialisations and namespace merging. Websocket. Server: :server. Websocket++’s basic logger allows us to set an ostream alternative for the standard std: :cout and std: :cerr. We will look at this in more detail later on. These are all callback methods we will define in a moment.
Get char string via e. This method is not exception- free, so we surround it with a try/catch block. We just need to start the server proper now, which is done in the following function. Websocket. Server: :run() . Get message via e.
The other clue here is that when we shut down the server at some point, we have to wait for this (blocking) run() call to return before we for example terminate a thread. Log reason using ec. Log reason using ec. This means we will no longer accept new connections. Next we go through all of the websocket connections we still have and close every single one of them.
Finally we call stop() on the server object. This isn’t strictly necessary, but it will ensure that the transport backend is completely shut down and any remaining connections forcefully terminated. Let’s move on to actually accepting new connections. For this we can use a number of handlers . I picked the validate handler, since it allows one to filter incoming connections and reject any which do not authenticate properly or such. Websocket. Server: :on.
We use pthread- based locking around the websockets map to ensure no concurrent access takes place on this data structure and insert the new websocket handle with its id as key. We may also wish to implement the fail() and close() handlers. Websocket. Server: :on. Log reason using ec.
Log reason using ec. The get. Websocket() method is a trivial STL map- based find and iteration effort and isn’t further documented here.
Do not forget to lock the map while performing said find and iterator actions on it. Lastly, how to close a socket.
Websocket. Server: :send. Close(string id) . Log reason using ec. We can send a close reason using a string, or just send an empty string. Finally the ID is erased from the websockets map and the now invalid connection handle with it. With this we have everything we need for the Websocket server, except for one thing: the redirecting of the logging output from Websocket++.
We saw earlier that we use the set. In the class declaration we saw this mysterious . For the actual streambuf implementation, one would use something like this.
Log. Stream : public streambuf . In the default implementation the default buffer overflows for every character written to the streambuf class and thus our overflow method is called for each character. Using a string as buffer, we capture each received character and check whether it is a newline character or not. If it is we have a complete line which we can then write to whatever logging functionality we use in our project. After this we empty the buffer string and continue with the new line. In conclusion, I must say that despite the effort it cost me to get a working integration of Websocket++ in my project, I do think it was worth it.
Technically it is a well- designed library with a lot of cool features and thanks to its template- based nature ease of expansion and configuration to fit different purposes. Its main weakness is simply the outdated, lacking and occasionally wrong documentation and examples. Hopefully this article will fix at least part of that problem.