get vtable size

[Basile B. via Digitalmars-d-learn]

On Sunday, 7 May 2017 at 04:01:43 UTC, Mike B Johnson wrote:
> how many elements(virtual functions) are in the __vptr?

You don't need the size. The index you get with 
__traits(virtualIndex) is always valid. 
(https://dlang.org/spec/traits.html#getVirtualIndex)

However as an exercise you can still count them:

- iterate all the member
- check each overload with __traits(isVirtualMethod)

---
template virtualMethodCount(T)
{
     size_t c()
     {
         size_t result;
         foreach (member; __traits(allMembers, T))
             static if (is(typeof(__traits(getMember, T, member))))
             {
                 foreach (overload; __traits(getOverloads, T , 
member))
                     result += __traits(isVirtualMethod, overload);
             }
         return result;
     }
     enum objectVmc = 4; // toHash, opCmp, opEquals, toString
     enum virtualMethodCount = c() - objectVmc;
}

unittest
{
     class Foo{}
     static assert( virtualMethodCount!Foo == 0);
     class Bar{void foo(){} int foo(){return 0;}}
     static assert( virtualMethodCount!Bar == 2);
     class Baz{private void foo(){} int foo(){return 0;}}
     static assert( virtualMethodCount!Baz == 1);
}
---