Error: cannot implicitly convert expression this.aa of type inout(string[string]) to string[string]

[Simen Kjærås]

On Thursday, 15 March 2018 at 11:18:48 UTC, Robert-D wrote:
> struct S {
> string[string] aa;
>
>     S dup() inout pure {
>     	return S(aa);
>     }
> }
>
> void main() {
> 	auto s = S(["": ""]);
>     s.dup();
> }
>
> Result:
>  Error: cannot implicitly convert expression this.aa of type 
> inout(string[string]) to string[string]
>
>
> I need help with the above program.


This is where things go wrong:
>     S dup() inout pure {

'inout' means that this function can keep the const, immutable or 
mutable status of the type on which the function is called. This 
means that an inout function has to treat the object as const, 
because otherwise the function would break the guarantees of 
immutable and const.

When using inout on a function, you always want to put inout on 
something else too - either a ref parameter or the return value. 
In your case, this works:

     inout(S) dup() inout pure {
     	return inout(S)(aa);
     }

--
   Simen