# Function as struct field **URL:** \ **Category:** Help\ **Tags:** language\ **Created:** [March 2, 2024, 1:29pm UTC](https://ziggit.dev/t/function-as-struct-field/3431 "2024-03-02T13:29:32Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![pjfichet](https://ziggit.dev/letter_avatar_proxy/v4/letter/p/a5b964/32.png) [@pjfichet](https://ziggit.dev/u/pjfichet)\ **Post date:** [March 2, 2024, 1:29pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/1 "2024-03-02T13:29:32Z")
Hello alls ! I’m writing code as a hobby, and learning zig writing little toys. For one of them, I was hoping to have a function as field of a struct. My difficulty is that the function will be unique to each instance of the struct. I would like to write something like this, but can’t find the correct way to do so: ```zig const std = @import("std"); const Event = struct { state: u8, text: []const u8, condition: fn(*Event) bool, }; pub fn main() void { const event = Event { .state = 1, .text = "blah blah", .condition = fn (self: *Event) bool { if (self.state == 1) { return true; } }, }; if (event.condition() == true) { std.debug.print("event: {}\n", .{event.text}); } } ``` Is there a proper way to express this in Zig 0.11.0 ? ---
**Author:** ![dee0xeed](https://ziggit.dev/letter_avatar_proxy/v4/letter/d/3ab097/32.png) [@dee0xeed](https://ziggit.dev/u/dee0xeed)\ **Post date:** [March 2, 2024, 1:34pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/2 "2024-03-02T13:34:17Z")
You need a function pointer: ```zig const funcPtr = *const fn(args...) void; f_ptr: funcPtr = &nameOfAFunc; ``` ---
**Author:** ![pjfichet](https://ziggit.dev/letter_avatar_proxy/v4/letter/p/a5b964/32.png) [@pjfichet](https://ziggit.dev/u/pjfichet)\ **Post date:** [March 2, 2024, 1:47pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/3 "2024-03-02T13:47:48Z")
Thank you ! I still can’t find how to express this within a struct… ```zig const std = @import("std"); 2 3 const funcPtr = *const fn(*Event) bool; 4 5 6 const Event = struct { 7 state: u8, 8 text: []const u8, 9 condition: funcPtr, 10 }; 11 12 fn myCondition(self: *Event) bool { 13 if (self.state == 1) { 14 return true; 15 } 16 return false; 17 } 18 19 pub fn main() void { 20 const event = Event { 21 .state = 1, 22 .text = "blah blah", 23 .condition = &myCondition, 24 }; 25 if (event.condition() == true) { 26 std.debug.print("event: {}\n", .{event.text}); 27 } 28 } ``` ```zig error: expected 1 argument(s), found 0 if (event.condition() == true) { ``` ---
**Author:** ![dee0xeed](https://ziggit.dev/letter_avatar_proxy/v4/letter/d/3ab097/32.png) [@dee0xeed](https://ziggit.dev/u/dee0xeed)\ **Post date:** [March 2, 2024, 2:01pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/4 "2024-03-02T14:01:19Z")
> [@pjfichet](#): > > to express this within a struct It does not matter whether a function pointer is a filed of a struct or a standalone variable, syntax is the same. ---
**Author:** ![dimdin](https://ziggit.dev/user_avatar/ziggit.dev/dimdin/32/1457_2.png) [@dimdin](https://ziggit.dev/u/dimdin)\ **Post date:** [March 2, 2024, 2:03pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/5 "2024-03-02T14:03:47Z")
```zig const std = @import("std"); const funcPtr = *const fn (*const Event) bool; const Event = struct { state: u8, text: []const u8, condition: funcPtr, }; fn myCondition(self: *const Event) bool { return (self.state == 1); } pub fn main() void { const event = Event{ .state = 1, .text = "blah blah", .condition = &myCondition, }; if (event.condition(&event)) { std.debug.print("event: {s}\n", .{event.text}); } } ``` ---
**Author:** ![pjfichet](https://ziggit.dev/letter_avatar_proxy/v4/letter/p/a5b964/32.png) [@pjfichet](https://ziggit.dev/u/pjfichet)\ **Post date:** [March 2, 2024, 2:07pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/6 "2024-03-02T14:07:54Z")
Thank you for the answers ! I’m a bit sad I can’t define the function while instanciating the struct, and use the syntactical sugar to call it without argument. But it will do ! ---
**Author:** ![tgirod](https://ziggit.dev/user_avatar/ziggit.dev/tgirod/32/3744_2.png) [@tgirod](https://ziggit.dev/u/tgirod)\ **Post date:** [March 2, 2024, 2:34pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/7 "2024-03-02T14:34:15Z")
Right now zig does not support anonymous functions - you have to define them “inside” something - struct, union, enum, or root level of a file. ---
**Author:** ![dee0xeed](https://ziggit.dev/letter_avatar_proxy/v4/letter/d/3ab097/32.png) [@dee0xeed](https://ziggit.dev/u/dee0xeed)\ **Post date:** [March 2, 2024, 2:37pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/8 "2024-03-02T14:37:10Z")
My little example, just in case (a sort of dynamic dispatch via virtual method table, with one element in this case): ```zig const std = @import("std"); const log = std.debug.print; const methodPtr = *const fn(c: *Class) void; const Class = struct { x: u32, m: methodPtr, fn callMethod(i: *Class) void { i.m(i); } }; pub fn main() void { var inst1 = Class{.x = 1, .m = &m1}; var inst2 = Class{.x = 2, .m = &m2}; inst1.callMethod(); inst2.callMethod(); } fn m1(i: *Class) void { log("my x is {}\n", .{i.x}); log("I can do this\n", .{}); } fn m2(i: *Class) void { log("my x is {}\n", .{i.x}); log("I can do that\n", .{}); } ``` Also, when I see the words “state” and “event” close to each other, I immediately start to guess that this is about “event driven state machines” or so. EDSM, as I call it, is a design methodology which I advocate here and there 🙂 If you are interested you can take a look at my GitHub repos with `edsm` in names (nick is the same as on this forum, dee0xeed). There are various versions of the engine and, If I remember right, the most recent was [here](https://github.com/dee0xeed/xjiss/tree/main/xjiss2/src/engine). ---
**Author:** ![dude\_the\_builder](https://ziggit.dev/user_avatar/ziggit.dev/dude_the_builder/32/557_2.png) [@dude\_the\_builder](https://ziggit.dev/u/dude_the_builder)\ **Post date:** [March 2, 2024, 3:45pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/9 "2024-03-02T15:45:18Z")
Just a little style note here: In all these examples, `funcPtr` and `methodPtr` are types so they should be PascalCase: `FuncPtr` and `MethodPtr`. Also, since they’re used only once, why not just use the type directly in the struct. field def: ```zig m: *const fn(c: *Class) void, ``` ---
**Author:** ![dee0xeed](https://ziggit.dev/letter_avatar_proxy/v4/letter/d/3ab097/32.png) [@dee0xeed](https://ziggit.dev/u/dee0xeed)\ **Post date:** [March 2, 2024, 5:03pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/10 "2024-03-02T17:03:04Z")
> [@dude\_the\_builder](#): > > are types so they should be PascalCase: `FuncPtr` and `MethodPtr` TBH, I do not know why I used camelCase for them, I depicted it from some code I wrote before (probably at that time I did not think right and was not realizing these should be in PascalCase). And, BTW, here is small excerpt from `/opt/zig-0.11/lib/std$ grep -rI "= \*const fn"`: ```zig os/plan9.zig: pub const handler_fn = *const fn (c_int) callconv(.C) void; os/plan9.zig: pub const sigaction_fn = *const fn (c_int, *const siginfo_t, ?*const anyopaque) callconv(.C) void; os/windows/ws2_32.zig:pub const LPCONDITIONPROC = *const fn ( ``` 😁 > [@dude\_the\_builder](#): > > why not just use the type directly in the struct. field def Because at a later time they may be used more than once, why not declare the type in advance? ---
**Author:** ![pjfichet](https://ziggit.dev/letter_avatar_proxy/v4/letter/p/a5b964/32.png) [@pjfichet](https://ziggit.dev/u/pjfichet)\ **Post date:** [March 2, 2024, 6:42pm UTC](https://ziggit.dev/t/function-as-struct-field/3431/11 "2024-03-02T18:42:35Z")
Thank you all for the detailed explanations. @dee0xeed : I will study event driven state machines and your implementation, as indeed, it might be a solution to do what I’m trying to do.