77 lines
2.5 KiB
Zig
77 lines
2.5 KiB
Zig
//
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// A union lets you store different types and sizes of data at
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// the same memory address. How is this possible? The compiler
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// sets aside enough memory for the largest thing you might want
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// to store.
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//
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// In this example, an instance of Foo always takes up u64 of
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// space in memory even if you're currently storing a u8.
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//
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// const Foo = union {
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// small: u8,
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// medium: u32,
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// large: u64,
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// };
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//
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// The syntax looks just like a struct, but a Foo can only hold a
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// small OR a medium OR a large value. Once a field becomes
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// active, the other inactive fields cannot be accessed. To
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// change active fields, assign a whole new instance:
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//
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// var f = Foo{ .small = 5 };
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// f.small += 5; // OKAY
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// f.medium = 5432; // ERROR!
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// f = Foo{ .medium = 5432 }; // OKAY
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//
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// Unions can save space in memory because they let you "re-use"
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// a space in memory. They also provide a sort of primitive
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// polymorphism. Here fooBar() can take a Foo no matter what size
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// of unsigned integer it holds:
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//
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// fn fooBar(f: Foo) void { ... }
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//
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// Oh, but how does fooBar() know which field is active? Zig has
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// a neat way of keeping track, but for now, we'll just have to
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// do it manually.
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//
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// Let's see if we can get this program working!
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//
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const std = @import("std");
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// We've just started writing a simple ecosystem simulation.
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// Insects will be represented by either bees or ants. Bees store
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// the number of flowers they've visited that day and ants just
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// store whether or not they're still alive.
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const Insect = union {
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flowers_visited: u16,
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still_alive: bool,
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};
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// Since we need to specify the type of insect, we'll use an
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// enum (remember those?).
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const AntOrBee = enum { a, b };
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pub fn main() void {
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// We'll just make one bee and one ant to test them out:
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var ant = Insect{ .still_alive = true };
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var bee = Insect{ .flowers_visited = 15 };
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std.debug.print("Insect report! ", .{});
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// Oops! We've made a mistake here.
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printInsect(ant, AntOrBee.a);
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printInsect(bee, AntOrBee.b);
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std.debug.print("\n", .{});
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}
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// Eccentric Doctor Zoraptera says that we can only use one
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// function to print our insects. Doctor Z is small and sometimes
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// inscrutable but we do not question her.
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fn printInsect(insect: Insect, what_it_is: AntOrBee) void {
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switch (what_it_is) {
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.a => std.debug.print("Ant alive is: {}. ", .{insect.still_alive}),
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.b => std.debug.print("Bee visited {} flowers. ", .{insect.flowers_visited}),
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}
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}
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