comptime Parameters and Values
Pass values that exist at compile time.
comptime Parameters and Values is a free Zig Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Zig Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Some Arguments Must Be Known Early
Sometimes a function needs an argument the compiler can see, not just the running program. Zig lets you mark such a parameter comptime. 🧩
Marking a comptime Parameter
Write comptime before a parameter name to require that its value be known at compile-time. The caller must pass a known value.
fn repeat(comptime n: usize) void {}The Caller Supplies a Known Value
A comptime parameter accepts only values the compiler already knows, like literals or other compile-time constants.
repeat(3);Run-Time Values Are Rejected
Pass a value that is only known at run-time into a comptime parameter and Zig reports a clear compile error.
comptime Inside a Body
You can also force a local expression to evaluate early. A comptime block runs during compilation and produces a constant.
const table = comptime buildTable();comptime var for Compile-Time State
A comptime var is a mutable variable that exists only during compilation. Use it to accumulate a value as compile-time code runs.
comptime var sum = 0;Known Values Specialize Functions
Because a comptime argument is fixed, Zig can generate a tailored version of the function for that exact value.
Mix comptime and Normal Parameters
A function can take both kinds at once: a comptime parameter for the known part and ordinary parameters for run-time data.
fn scale(comptime factor: u8, x: u32) u32 {
return x * factor;
}Validate Arguments Early
Since the value is known, you can check it during compilation and stop the build if it is wrong, before the program ever runs. ✅
No Run-Time Cost
Work driven by a comptime parameter happens during the build. The resulting binary carries the answer with nothing left to compute.
Inside, the Parameter Is Constant
Within the function body a comptime parameter behaves like a fixed constant, so you may use it in array sizes and other compile-time spots.
fn buffer(comptime n: usize) [n]u8 {
return undefined;
}Quick Check
A function declares a parameter as comptime n: usize. What kind of value must the caller pass?
Recap
Mark a parameter comptime to demand a compile-time value. Zig can then validate it and specialize the function with no run-time cost. 🎯
Frequently asked questions
Is the “comptime Parameters and Values” lesson free?
Yes — the full text of “comptime Parameters and Values” is free to read here on the web, and the Zig Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Zig Academy course, upgrade to CoddyKit PRO.
What will I learn in “comptime Parameters and Values”?
Pass values that exist at compile time. You practise Zig Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Zig Academy?
No prior experience is required. Zig Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “comptime Parameters and Values” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Zig Academy lesson?
Yes. Every Zig Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Compile-Time vs Run-Time
- comptime Parameters and Values
- Types Are comptime Values
- inline for and Unrolled Loops