Suppose that I have two struct templates which take identical parameter lists:

    struct Foo(T1, T2, T3)
    {
        ...
    }

    struct Bar(T1, T2, T3)
    {
        ...
    }

Now suppose that I have a Foo which has been instantiated with a given set of parameters.  Is there any way for me to say, "now instantiate a Bar with the same parameters?"

The use-case I'm thinking of is a function something like this (somewhat pseudo-code-y):

    auto fooToBar(FooInstance f)
    {
        Bar!(f.T1, f.T2, f.T3) b;
        // set values etc.
        return b;
    }

Of course the f.T1 notation is my fiction, but it gives the idea of what is needed -- is there a means to extract and use template parameters in this way? I assume something from std.traits but it's not entirely clear what or how ...

On Tuesday, 6 November 2012 at 15:20:43 UTC, Joseph Rushton Wakeling wrote:

> Of course the f.T1 notation is my fiction, but it gives the idea of what is needed -- is there a means to extract and use template parameters in this way? I assume something from std.traits but it's not entirely clear what or how ...

Would something like this be what you are after?

import std.stdio;

struct Foo(_T1, _T2, _T3)
{
  alias _T1 T1;
  alias _T2 T2;
  alias _T2 T3;
}

struct Bar(_T1, _T2, _T3)
{
  alias _T1 T1;
  alias _T2 T2;
  alias _T2 T3;
}

auto fooToBar(F)(F f) {
  Bar!(F.T1, F.T2, F.T3) b;
  writeln("I have a ", typeid(typeof(b)));
  return b;
}
void main() {
  alias Foo!(int, string, char) X;
  X x;
  fooToBar(x);
}

Thanks
Dan

On 11/06/2012 04:34 PM, Dan wrote:
> Would something like this be what you are after?

Yes!  I actually had tested something similar, but as I was using the variable name when trying to extract the parameter, it didn't work.

Thanks very much!