Swift regards a C const pointer as mutable?

Question

I'm importing a C header in Swift using a bridging header. My C API looks something like this:

typedef struct MyStruct {
  char buff[80];
} MyStruct;

const char* GetBuff(const MyStruct* m);

Now in Swift I try to call it like this:

let b = MyStruct()
let mystr = String(cString: GetBuff(&b))

Then I get this compile error: "Cannot pass immutable value as inout argument: 'b' is a 'let' constant". Why is this? The argument to GetBuff() is a const pointer. Isn't a const pointer immutable?

I know that changing 'b' to 'var b' will fix the problem but why is it necessary?

[`const char*` — is a `mutable` pointer to `const char`](https://cdecl.org/?q=const+char*x). Maybe you're looking for `char * const`, or `const char * const`? — user28434'mstep, Feb 22 '20 at 12:11
Relevant: https://stackoverflow.com/questions/1446120/const-usage-with-pointers-in-c — user28434'mstep, Feb 22 '20 at 12:14
@user28434: The `char *` return type is not the problem here. — Martin R, Feb 22 '20 at 12:23

Martin R · Accepted Answer · 2020-02-22T12:29:32.590

0

let b is a constant, therefore you cannot pass it as inout argument with &b. But you can use withUnsafePointer(to:) to obtain a (temporary) pointer to its value and pass that to the C function.

let b = MyStruct()
let mystr = withUnsafePointer(to: b) { String(cString: GetBuff($0)) }

edited Feb 22 '20 at 12:29

answered Feb 22 '20 at 12:17

Martin R

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Thanks! But why can't I pass its address? 'b' is a constant... – bear_dk Feb 22 '20 at 12:47
2

@bear_dk: `&b` in Swift is an “in-out-expression” to indicate that it can be modified in the function call. It cannot be used with `let` constants. Also constants in Swift do not necessarily have an address. – Martin R Feb 22 '20 at 12:56

Swift regards a C const pointer as mutable?

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