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digitalmars.D.learn - Issue with const

reply Jacob Carlborg <doob me.com> writes:
Say I have type and a function look like this:

class Foo
{
     void* data;

     Foo clone ()
     {
         auto c = new Foo;
         c.data = data;

         return c;
     }
}

void bar (Foo foo)
{
     auto c = foo.clone();
     ...
}

Since I'm not changing anything on "foo" I thought that it could be a 
good idea to declare it as const.

void bar (const Foo foo)
{
     auto c = foo.clone();
     ...
}

In this case I also have to make "clone" const as well. Which is fine 
sine "clone" doesn't change any of the fields.

class Foo
{
     void* data;

     Foo clone () const
     {
         auto c = new Foo;
         c.data = data;

         return c;
     }
}

But now when I compile this code I get this error:

Error: cannot implicitly convert expression (this.data) of type 
const(void*) to void*

Any idea how to solve this? Or would I need to drop const.

-- 
/Jacob Carlborg
Apr 09 2012
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 04/09/2012 04:49 PM, Jacob Carlborg wrote:

 But now when I compile this code I get this error:

 Error: cannot implicitly convert expression (this.data) of type
 const(void*) to void*

 Any idea how to solve this? Or would I need to drop const.

 --
 /Jacob Carlborg


Either clone the data too or use inout.
Apr 09 2012
parent reply Jacob Carlborg <doob me.com> writes:
On 2012-04-09 16:52, Timon Gehr wrote:

 On 04/09/2012 04:49 PM, Jacob Carlborg wrote:

 But now when I compile this code I get this error:

 Error: cannot implicitly convert expression (this.data) of type
 const(void*) to void*

 Any idea how to solve this? Or would I need to drop const.

 --
 /Jacob Carlborg


 Either clone the data too or use inout.


Where would I use inout, on "clone"?

-- 
/Jacob Carlborg
Apr 09 2012
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 09 Apr 2012 11:22:54 -0400, Jacob Carlborg <doob me.com> wrote:


 On 2012-04-09 16:52, Timon Gehr wrote:

 On 04/09/2012 04:49 PM, Jacob Carlborg wrote:

 But now when I compile this code I get this error:

 Error: cannot implicitly convert expression (this.data) of type
 const(void*) to void*

 Any idea how to solve this? Or would I need to drop const.

 --
 /Jacob Carlborg


 Either clone the data too or use inout.


 Where would I use inout, on "clone"?


// untested
inout(Foo) clone() inout {
    return new inout(Foo)(data);
}

Note, you can't post-assign data, since inout is effectively const inside  
an inout function.

-Steve
Apr 09 2012
parent reply Jacob Carlborg <doob me.com> writes:
On 2012-04-09 17:30, Steven Schveighoffer wrote:


 // untested
 inout(Foo) clone() inout {
 return new inout(Foo)(data);
 }

 Note, you can't post-assign data, since inout is effectively const
 inside an inout function.

 -Steve


Ok, that works. But then I want to modify the clone:

void bar (const Foo foo)
{
     auto c = foo.clone();
     c.data = ...
}

But now "c" is const. I don't want "c" to be const, I just want "foo" to 
be const.

-- 
/Jacob Carlborg
Apr 09 2012
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 09 Apr 2012 13:51:17 -0400, Jacob Carlborg <doob me.com> wrote:


 On 2012-04-09 17:30, Steven Schveighoffer wrote:


 // untested
 inout(Foo) clone() inout {
 return new inout(Foo)(data);
 }

 Note, you can't post-assign data, since inout is effectively const
 inside an inout function.

 -Steve


 Ok, that works. But then I want to modify the clone:

 void bar (const Foo foo)
 {
      auto c = foo.clone();
      c.data = ...
 }

 But now "c" is const. I don't want "c" to be const, I just want "foo" to  
 be const.


Then c.data cannot be the same reference as foo.data.

Counter-case:

void bar( const Foo foo)
{
    auto c = foo.clone(); // assume this works;
    *(cast(int*)c.data) = 6; // note even though I'm casting, there is no  
removal of const, so this should be defined behavior.
}

immutable int i = 5;
const foo = new Foo(&i);
bar(foo);

assert(i == 5); // oops, it's 6!

In that case, you have to clone the data as well.

-Steve
Apr 09 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-04-09 19:56, Steven Schveighoffer wrote:


 Then c.data cannot be the same reference as foo.data.

 Counter-case:

 void bar( const Foo foo)
 {
 auto c = foo.clone(); // assume this works;
 *(cast(int*)c.data) = 6; // note even though I'm casting, there is no
 removal of const, so this should be defined behavior.
 }

 immutable int i = 5;
 const foo = new Foo(&i);
 bar(foo);

 assert(i == 5); // oops, it's 6!

 In that case, you have to clone the data as well.

 -Steve


Ok, I see, thanks.

-- 
/Jacob Carlborg
Apr 09 2012
prev sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Monday, April 09, 2012 19:51:17 Jacob Carlborg wrote:

 On 2012-04-09 17:30, Steven Schveighoffer wrote:

 // untested
 inout(Foo) clone() inout {
 return new inout(Foo)(data);
 }
 
 Note, you can't post-assign data, since inout is effectively const
 inside an inout function.
 
 -Steve


 
 Ok, that works. But then I want to modify the clone:
 
 void bar (const Foo foo)
 {
 auto c = foo.clone();
 c.data = ...
 }
 
 But now "c" is const. I don't want "c" to be const, I just want "foo" to
 be const.


I don't see how that's possible with a void* member variable. clone can be 
const as long as you actually deep copy all of foo's member variables. If 
there are any that you don't deep copy, then the fact that clone is const 
forces the return value to be const because of transivity. And with a void* 
member variable, I don't see how you could deep copy it, so the void* member 
variable is stuck as const, making the return value stuck as const.

- Jonathan M Davis
Apr 09 2012