Embedding raw C++

jank has a special cpp/raw form which accepts a single string containing literal C++ code. This can be used for bringing in pretty much anything. For example, we can use this to include header files.

(cpp/raw "#include <fstream>")

jank will always compile the included C++ source in a global scope, even if you put the cpp/raw form within a nested scope, such as within a function or a let. For example, this code will have the same effect, even if this function is never called.

(defn foo []
  (cpp/raw "#include <fstream>"))

The cpp/raw form always evaluates to nil. At runtime, foo will do nothing but return nil, since the JIT compilation is where the effect of cpp/raw actually happens.

A helpful idiom

Hopefully this becomes a less common idiom simply by not being needed, but for now it’s common enough. If you run into issues trying to access a member, call a function, etc using normal C++ interop, you can write a wrapper in cpp/raw which will do the trick. For example, let’s say we have the following code.

(let [s (cpp/std.string)
      ; Let's say that this interop call isn't compiling correctly, due to a
      ; jank bug.
      size (cpp/.size s)]
  (println "The size is" size))

You can work around this issue by defining a helper function which does the C++ work for you. In this case, we could do the following.

(cpp/raw "size_t get_string_size(std::string const &s)
          { return s.size(); }")

(let [s (cpp/std.string)
      size (cpp/get_string_size s)]
  (println "The size is" size))

Of course, if you need to use this, please also report a bug on jank’s Github which describes what you tried to do and why it didn’t work.