Problem with taking inout, const references

[Ali Çehreli]

On 03/26/2014 12:17 PM, Uranuz wrote:

 > I modified this with casting pointer to pointer to *inout* and it
 > compiles then.
 >
 > inout(Cookie)* opBinaryRight(string op)(string name) inout if(op == "in")
 > {    foreach( ref inout(Cookie) c; _cookies )
 >          if( c.name == name )
 >              return cast(inout(Cookie)*) &c; //Error is here
 >      return null;
 > }

I think it is a bug then. I only now understand the problem. Here is a 
reduced case:

class C
{
     int[1] arr;

     inout(int)* foo() inout
     {
         foreach (ref e; arr) {
             static assert(is (typeof(e) == inout(const(int)))); // WAT?
             return &e;
         }
     }
}

void main()
{
     (new C).foo();
}

The problem is, when the foreach variable is ref, the type gains a const 
as well. Unless there is a reason for that we should file a bug.

Another workaround for you is using the foreach index:

     {    foreach(i, ref inout(Cookie) c; _cookies ) // NOTE i
             if( c.name == name )
                 return &_cookies[i];                // NOTE i
         return null;
     }

 > As I think compiler may be should create const version and mutable
 > version of this function.

Type is a compile-time concept; there is no difference between the 
compiled mutable versus const versions of your inout function. So, there 
is no need for more than one compilation.

 > In case when it's mutable it should return pointer to mutable
 > pointer to *const* data when function is const.

Makes sense and it should be checked at compile time.

 > I don't understand why compiler produce pointer to const,

I think that's a bug.

 > In case when all is treated like const

The body of an inout function must not mutate the object because the 
same body supports the non-mutable cases as well. So, it is acceptable 
for the compiler to compile as if it's const. However, as you say, the 
interface of the function still obeys the actual types used in the program.

You are right. I think it is just a bug.

Ali