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Discussion on: When a var really isn't a var (ft. String Literals)

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gowind profile image
Govind Author • Edited on

Thanks for the reply !
The Zig specific behaviour of comptime_int makes more sense , but for strings I can still do var input = "Hello Zig".* and then modify it to something like input[0] = 'a' and input actually changes to aello Zig. What happens in this case?
Does Zig create a copy of Hello Zig on the stack, or does it create a completely new String and then modified that ?

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kristoff profile image
Loris Cro • Edited on

"Hello Zig" is a chunk of memory inside the .rodata of your executable (or something of that sort, I believe different architecture-specific backends can decide where this stuff goes). So when you assing it to a variable, you get a pointer to those bytes.

When you dereference the pointer you get the full array contents which, yes, get copied to stack memory (assuming we're inside a function) and that then you can modify, since that memory is yours. If you look at the types it's very clear and consistent (if you know about string interning).

var str_ptr = "hi"; // type: *const [2:0]u8
var a = str_ptr.*; // type: [2:0]u8 
var b = [2:0]u8 {'y', 'o'}; // type: same as `a`
a = &b; // allowed since `b`'s type matches and `a` is var
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gowind profile image
Govind Author

When you dereference the pointer you get the full array contents which, yes, get copied to stack memory (assuming we're inside a function) and that then you can modify, since that memory is yours

Ok, this is the context I was missing (basically dereferring "x".*) creates a copy on the Stack that I can then modify. Thanks !