Recap: CTFE

We first introduced Compile Time Function Evaluation in session 2. It means precisely what its name implies: the compiler evaluates the result of a function whose parameters are known at compile time. However, as we will see in this session, the compiler can also generate additional code, apart from executing it.

static and enum

The keyword that is mostly associated with CTFE is static. In session 2 you learnt about static ifs. They are conditions just like regular ifs that are not verified at runtime, but at compile time. This has the following advantages:

• running times are lower because the compiler now picks up some of the work
• they allow the compiler to make flexible and complex decisions about what code to compile and what code to ignore

However, these perks come at a cost: all conditions must be evaluatable at compile time. This means that all values and types must be known by the compiler. Therefore, for example, no user input or config file data can be used in a static if.

Demo

Let’s see what the static keyword is all about. We’ll use the code in demo/ctfe/ctfe.d for this. The first unittest contains 2 static asserts. The only difference from regular asserts is that the compiler verifies them when compiling with the flag -unittest, instead of us running the resulting executable.

Now look at the next unittest if statements inside the unittest. Their body contains a pragma(msg, ...) statement, which prints a message to standard output at compile time. Now compile and run the code if you haven’t done so already.

Quiz

Remember that if the condition of a static if is not satisfied, the compiler simply ignores whatever is inside it… Well, not entirely. The body of the static if must still be syntactically correct, but not semantically correct. Now it should be obvious that the compiler does not generate code for this static if body. This is important for optimisations or for deciding what code to emit based on compile-time conditions.

Practice

Uncomment the code in the last unittest and modify it so that the test passes.