ziglings/exercises/067_comptime2.zig

65 lines
2.2 KiB
Zig
Raw Permalink Normal View History

2021-04-15 20:58:12 -04:00
//
2021-04-16 22:15:27 -04:00
// We've seen that Zig implicitly performs some evaluations at
// compile time. But sometimes you'll want to explicitly request
// compile time evaluation. For that, we have a new keyword:
2021-04-15 20:58:12 -04:00
//
2021-04-16 22:15:27 -04:00
// . . . o . . * . . .
// . * | . . . . . . * . .
// --o-- comptime * | .. .
// * | * . . . . --*-- . * .
// . . . . . . . . . | . . .
2021-11-05 11:44:29 -04:00
//
2021-04-16 22:15:27 -04:00
// When placed before a variable declaration, 'comptime'
// guarantees that every usage of that variable will be performed
// at compile time.
2021-04-15 20:58:12 -04:00
//
2021-04-16 22:15:27 -04:00
// As a simple example, compare these two statements:
2021-04-15 20:58:12 -04:00
//
// var bar1 = 5; // ERROR!
// comptime var bar2 = 5; // OKAY!
//
2021-04-16 22:15:27 -04:00
// The first one gives us an error because Zig assumes mutable
// identifiers (declared with 'var') will be used at runtime and
// we have not assigned a runtime type (like u8 or f32). Trying
// to use a comptime_int of undetermined size at runtime is
// a MEMORY CRIME and you are UNDER ARREST.
2021-04-15 20:58:12 -04:00
//
2021-06-12 18:15:19 -04:00
// The second one is okay because we've told Zig that 'bar2' is
// a compile time variable. Zig will help us ensure this is true
2021-04-16 22:15:27 -04:00
// and let us know if we make a mistake.
2021-04-15 20:58:12 -04:00
//
const print = @import("std").debug.print;
2021-11-05 12:37:12 -04:00
pub fn main() void {
2021-04-15 20:58:12 -04:00
//
// In this contrived example, we've decided to allocate some
2021-04-16 22:15:27 -04:00
// arrays using a variable count! But something's missing...
2021-04-15 20:58:12 -04:00
//
2024-06-05 21:01:52 -04:00
comptime var count = 0;
2021-04-15 20:58:12 -04:00
count += 1;
const a1: [count]u8 = .{'A'} ** count;
2021-04-15 20:58:12 -04:00
count += 1;
const a2: [count]u8 = .{'B'} ** count;
2021-04-15 20:58:12 -04:00
count += 1;
const a3: [count]u8 = .{'C'} ** count;
2021-04-15 20:58:12 -04:00
count += 1;
const a4: [count]u8 = .{'D'} ** count;
2021-04-15 20:58:12 -04:00
2021-11-05 12:37:12 -04:00
print("{s} {s} {s} {s}\n", .{ a1, a2, a3, a4 });
2021-04-16 22:15:27 -04:00
// Builtin BONUS!
//
// The @compileLog() builtin is like a print statement that
// ONLY operates at compile time. The Zig compiler treats
// @compileLog() calls as errors, so you'll want to use them
// temporarily to debug compile time logic.
//
// Try uncommenting this line and playing around with it
// (copy it, move it) to see what it does:
//@compileLog("Count at compile time: ", count);
2021-04-15 20:58:12 -04:00
}