• Arne Ludwig (12/12) May 08 2014 Hello,
• anonymous (23/35) May 09 2014 I answered a similar question on stackoverflow [1]. The same

"Arne Ludwig" <ludwig.arne gmail.com> writes:

Hello,

when using opApply it seems natural to have two versions: one 
normal and one const. My problem is that I cannot find a way to 
describe both versions with one code block. Since there could be 
a number of basic variants with different numbers of delegate 
arguments this can lead to serious code duplication.

This problem was discussed years ago:

http://www.digitalmars.com/d/archives/digitalmars/D/opApply_and_const_63436.html

Small example:

http://pastebin.com/kRrPp6Yg

In that example I need four times (mostly) the same code. There 
should be any way around that. Has someone ideas?

"anonymous" <anonymous example.com> writes:

On Friday, 9 May 2014 at 05:26:12 UTC, Arne Ludwig wrote:
 Hello,

 when using opApply it seems natural to have two versions: one 
 normal and one const. My problem is that I cannot find a way to 
 describe both versions with one code block. Since there could 
 be a number of basic variants with different numbers of 
 delegate arguments this can lead to serious code duplication.

 This problem was discussed years ago:

 http://www.digitalmars.com/d/archives/digitalmars/D/opApply_and_const_63436.html

 Small example:

 http://pastebin.com/kRrPp6Yg

 In that example I need four times (mostly) the same code. There 
 should be any way around that. Has someone ideas?

I answered a similar question on stackoverflow [1]. The same
approach can be used here.

If dg doesn't mutate, the mutable overloads are de-facto const.
So, if you have a const dg, it's safe to cast the object's const
away and call the mutable versions of opApply:

    /* de-facto const if dg doesn't mutate */
    int opApply(int delegate (T*) dg) {
      ... implementation ...
    }

    /* ditto */
    int opApply(int delegate (size_t, T*) dg) {
      ... implementation ...
    }

    int opApply(int delegate(const T*) dg) const {
      return (cast() this).opApply(cast(int delegate(T*)) dg);
    }

    int opApply(int delegate (size_t, const T*) dg) const {
      return (cast() this).opApply(cast(int delegate(size_t, T*))
dg);
    }

[1]
http://stackoverflow.com/questions/22442031/how-to-make-a-template-function-const-if-the-template-is-true/22442425