Is possible to changed a function argument that was passed by value mid function by changing the original variable

[lassade]

Zig Version

0.14.0-dev.1298+d9e8671d9

Steps to Reproduce and Observed Behavior

Minimal test repo code

const std = @import("std");

const Node = extern struct {
    id: u32 = 0,
    kind: u16 = 0,
    ver: u16 = 0,
};

const Context = struct {
    visitor: *Vistor,
};

const Vistor = struct {
    selected: Node = .{},
};

fn visit(target: Node, context: anytype) void {
    if (target.id == 0 or target.kind == 0 or target.ver == 0) {
        @panic("`target` is not invalid"); // will never happen
    }

    context.visitor.selected = .{}; // invalidate selected

    if (target.id == 0 or target.kind == 0 or target.ver == 0) {
        @panic("`target` changed it's value, but I thought it was passed by value not by ref!");
    }

    // can't continue because the target value changed!
}

test {
    var visitor = Vistor{};
    visitor.selected = .{ .id = 8, .kind = 1, .ver = 9 }; // starts with a valid target

    var context = Context{ .visitor = &visitor }; // create a context

    const temp = visitor.selected; // copy selected into a temp variable just to be sure
    visit(temp, &context);
}

Expected Behavior

I wasn't expecting the value of target to change mid function because I changed other variable. The only explanation is that zig is not passing it by value!

[squeek502]

Related / possible duplicate: #5973

[rohlem]

The only explanation is that zig is not passing it by value!

You are correct, and this is somewhat documented: https://ziglang.org/documentation/master/#Pass-by-value-Parameters
A workaround in status-quo is to make an explicit stack copy const x = y; and then use that copy instead.

The current documentation is missing details on what is / isn't allowed to happen here,
and the current implementation is missing a safety check in safe build modes (#2301).

[lassade]

The stack copy doesn't work, unless I force it some how to be var instead of const, but not sure if this a reliable workaround in other build modes