How does it work - Meta macros21 Mar 2014
Meta macros are pretty nifty but trying to follow how they work can really challenge the limits of your mental stack frame.
Today I'm going to try and follow macro expansion from start to finish using libextobjc to do some basic metaprogramming. This is going to be a bumpy ride but as always it's great to see how different techniques can be used to solve problems.
Let's start with a fictitious problem that I would like to solve with some metaprogramming. I would probably never do this in a real project but it gives me a realistic use case to work through.
Imagine I want my view controllers to fail hard if I forget to connect up an outlet in the xib file. I could start with something like this:
whoa that's a lot of repetition and it's not going to scale well. What would be great is if I could write some code that would write this repetitious code for me, ideally I would just type something like this:
This seems a lot DRY'er so let's aim for something similar to this and see how we get on.
Down the rabbit hole we go
After examining the metamacros header I can see that there is a foreach macro that sounds like it would be perfect for this task.
The definition of
metamacro_foreach looks like this:
After reading the docs I can see that the
MACRO argument should be the name of a macro that takes two arguments in the form of
MACRO(INDEX, ARG). The
INDEX parameter will be the index of the current iteration in the for loop and the
ARG parameter will be the argument for the current iteration in the for loop.
So I need to start of by defining a macro that takes these two arguments and expands to the
NSParameterAssert that I want. Here's a first stab at such a macro
I don't actually care to use the value of
INDEX so it is ignored. This is the macro that will be used within the
metamacro_foreach and will eventually expand into the required
In each of the following examples I'll show the input (starting macro) above the 3 dashes and what this would theoretically expand into below the 3 dashes. I'll optionally show any macro definitions at the top of the code block.
Here's how my
OUTLET_ASSERT macro will work:
Now let's see how we can use
metamacro_foreach to write the
PASAssertConnections macro that will take in a list of ivar names and expand them to the required
In this case I pass
OUTLET_ASSERT as the macro to use on each iteration. I pass
; to use as a separator between iterations, which will terminate each
NSParameterAssert. Then finally a comma separated list of ivar names that we are going to iterate over and generate the
With the previous expansion there are now two new macros that we need to look up and understand
metamacro_foreach_iter is arguably the simpler of the two but it's not needed until the end so let's see how
metamacro_concat is the easier of the two so we'll take a look at that first.
Now we need to jump back to see how
metamacro_argcount macro uses another macro called
metamacro_at is similar in concept to indexing into an array like
myArray[index]. In plain English this macro is the same as "give me the
nth item in the following list".
metamacro_argcount macro uses a clever little trick. If we put the numbers from
INDEX down to
0 into an array and then ask for the value at
INDEX we would get the last number, which would be 0. If we preprend something to the beginning of this array and asked for the value at
INDEX again we would now get
Let's see this in Objective-C so it's easier to picture:
The relationship is that when you prepend an argument to the array you shift all of the numeric values to the right by one step, which moves a higher number into the index that is being fetched. This of course only works up to the value of
INDEX - so we can tell that this particular implementation of metamacros only supports 20 arguments.
NB - this implementation of metamacros requires at least one argument to be given when using
You'll see the trick of inserting
__VA_ARGS__ into argument lists at different points used a few times so it's worth making sure you understand what is happening above.
Ok so that makes sense but what about
Great there's our old friend
metamacro_concat so we don't need to look up how that works again to know that this will expand like this:
The change is very subtle. The
20 has moved from being an argument to now actually being part of the macro name. So now we need to look up
It turns out that there are variants of
metamacro_at defined for
20, which allows you to access any of the first 20 arguments from the
This is another common trick you'll see with metamacros, at some point you have to knuckle down and write out multiple versions of the same macro to handle different length argument lists. You'll often see that metamacros are generated by other scripts that allow you to specify how many arguments you would like to support without having to hand roll all the variations of
metamacro_at a little easier to digest I'll examine one of the smaller versions of this macro.
_1 arguments are basically used as placeholders to gobble up the items at indices
1 from the arguments. Then we bundle the rest of the arguments together with
.... The newly trimmed
__VA_ARGS__ is then passed into
metamacro_head uses the opposite trick to
metamacro_at*. In this case we are only interested in the first item and we want to throw away the rest of the
__VA_ARGS__ list. This is achieved by grabbing the first argument in
FIRST and then collecting the rest with
Wow that escalated quickly. We now need to unwind out mental stack frame back to
Don't worry the end is now very much in sight, just a couple more painless macro expansions. The previous expansion gives us the new
metamacro_foreach_cxt2 macro to check out.
This is another example of macro that has multiple versions defined from
0..20. Each of these foreach macros works by utilising the foreach macro that is defined to take one less argument than itself until we get all the way down to
We are now at the point where we need to see what
MACRO expands to. In this case
MACRO is actually the
metamacro_foreach_iter macro that we passed in near the beginning and I delayed explaining.
This macro is really just an implementation detail and as such shouldn't be used directly but we still want to see what part it plays:
Nice and simple -
metamacro_foreach_iter is just a helper that takes our macro
OUTLET_ASSERT and the two arguments that our macro should receive and puts the pieces in the right order to be further expanded into the
NSParameterAssert calls that we want.
Thankfully that was only a minor detour so let's get right back to
If you have gotten this far then the above is nothing special so we can progress straight to the next step:
And that's it - we've followed the
metamacro_foreach macro from the beginning of it's use all the way to it's end expansion and hopefully our heads are still in one piece.
At the beginning of the post I said I was aiming for
PASAssertConnections(firstNameTextField, lastNameTextField, passwordTextField, passwordConfirmation);
now I'm actually one step away from achieving this, but if this post has gotten your interest I'll leave that as a simple exercise - it's always better to learn by doing and not just skimming through blog posts hoping to learn by osmosis.
Metaprogramming is normally something that people associate with more dynamic languages like Ruby but there's a whole load of possibilities and cool tricks out there just waiting to be learned. As always I encourage you to join me in peeling back the curtain and seeing that there is normally no magic to be found in your favorite OSS projects.