Emacs Custom Compilation

One of the convenient things I quite like about Sublime Text is the easy to setup build system¹. It is pretty easy to add a custom build setup with Sublime Text, to compile a single C++17 source file, for example. Once configured it is very efficient and keyboard friendly to compile and run a C++ program with a single key press, F7 with default key bindings.

This blog post is about how to configure Emacs in a similar vein.

The use case I had in mind was to configure Emacs to compile and run single file C++ programs without having to write a Makefile. It is pretty easy to invoke shell commands within Emacs, so if you have a build setup with make and the like, it is not difficult to to build and run programs within Emacs. However, what I wanted was to build and run one off C++ source files.

Compile Command

Emacs compile command to the rescue²!

Default compile command for C/C++ mode is make -k, which is not what I want in this case. Compile command can be customised on a per-mode basis.

Following is the compile command I configured for compiling one off C++ source files. This command builds and runs C++ source files based on the current buffer name. For our purposes, buffer name is equivalent to the file name of the C++ source file.

(add-hook 'c++-mode-hook
          (lambda ()
            (set (make-local-variable 'compile-command)
                 (format "g++ -std=c++17 -Wall %s -o a.out && ./a.out" (shell-quote-argument (buffer-name))))))

• We are supplying the file name which can be found out from buffer-name elisp function as the input file parameter for g++ compiler. Note that this command will work on macOS as well.
• We need to quote the file name to be run as a shell command, which is what the shell-quote-argument does.

Once setup up, to compile the current source file you are editing can be accomplished by invoking compile command in Emacs (M-x compile).

Since the compile command is differentiated based on the mode it is bound to, you can have custom compilation commands for different programming languages.

Per-file (or per-directory) customisation

Pretty cool thing about the compile-command is that it can be set and customised on a per-file basis, using Emacs file variables³. These variables are set when visiting a file, or when setting the major of a buffer, and makes them local to the buffer.

Following is an example local variable specification on a C++ file, customising the executable name, C++ version, etc. Make sure to quote compile command as a string. Emacs makes it pretty easy to add them as well - use make-local-variable⁴ command and Emacs will insert the right type of commented variable declaration automatically!

// Local Variables:
// compile-command: "g++ -std=c++11 -Wall hello-world.cpp -o hello && ./hello"
// End:

Similar customisations can be applied on a directory basis with .dir-local.el files⁵. With directory local variables, you can set the compile command on a project basis, instead of relying on the global configuration as shown with the add-hook method, or having to specify on a per-file basis as shown in the example above.

As can be seen from above examples, Emacs offers a couple of ways to customise builds. Emacs’ malleability reminds me of the following classic xkcd⁶!

References