How to Statically Initialize Arrays with Arbitrary Values

[Warning: Low-level C stuff ahead!]

Imagine a situation where you want to statically initialize an array with values different to 0:

This approach works, at least until someday you want to increase the array size to, say, 200. In this case, you have to add 192 times “42, ” to the initializer list. What a dread!

Everything would be easy, if you wanted to zero-initialize the array:

With zero-initialization, all you have to do is specify the value of the first element – all of the remaining elements will automatically be set to zero.

But sometimes you need a value different to 0 and you don’t want an additional call to “memset()” at run-time. Or you cannot use “memset()” because your array is stored in a read-only ROM segment and you cannot change the array’s values dynamically.

Basically, what you want is this:

Then, you would only have to make a single change to alter the size of the array (or the initialization value).

Alas, it turns out that it is impossible to define a macro that does the job we expect from “STATIC_INIT”. Think about it for a while. How would you solve this problem?

Sometimes, it is possible to replace a call to an impossible macro with the inclusion of a header file; I call this technique the “Replace macro call with file inclusion” trick:

The two defines represent the macro parameters and the inclusion of the header file represents the actual macro call.

You probably wonder what the contents of “static_init.h” are, but it’s instructive to spend some time on this problem yourself. Afterwards, you can have a look at my solution.

Note that this approach is not limited to single values – you can also use it for more complex initializations. For instance, if you need an alternating sequence of ’42’ and ’13’ you would do this:

I’ve also used the “Replace macro call with file inclusion” trick to encapsulate #pragmas and other compiler-specific features. Consider the case where you are working on a multi-platform project that uses different compilers. Consider further that you have a piece of code that generates compiler warnings and you want to locally turn compiler warnings off:

Now the problem with this approach is that #pragmas are compiler-dependent, which means that you will end up with something like this:

Not only does this litter the code – it is also a maintenance nightmare.

Usually, the solution is to encapsulate compiler specific features in #defines; alas this obvious strategy doesn’t work for #pragmas:

So it is time to roll out our trick once again:

Where, for instance, “warnings_off” looks like this:

You probably won’t need this trick very often, but when you do, it is good to know that it’s there.