Converting std::vector<>::iterator to .NET interface in C++/CLI

Question

I am wrapping a native C++ class, which has the following methods:

class Native
{
    public:
    class Local
    {
        std::string m_Str;
        int m_Int;
    };

    typedef std::vector<Local> LocalVec;
    typedef LocalVec::iterator LocalIter;

    LocalIter BeginLocals();
    LocalIter EndLocals();

    private:
        LocalVec m_Locals;
};

1) What is the ".NET way" of representing this same kind of interface? A single method returning an array<>? Does the array<> generic have iterators, so that I could implement BeginLocals() and EndLocals()?

2) Should Local be declared as a value struct in the .NET wrapper?

I'd really like to represent the wrapped class with a .NET flavor, but I'm very new to the managed world - and this type of information is frustrating to google for...

Answer 1

Iterators aren't exactly translatable to "the .net way", but they are roughly replaced by IEnumerable < T > and IEnumerator < T >.

Rather than

  vector<int> a_vector;
  vector<int>::iterator a_iterator;
  for(int i= 0; i < 100; i++)
  {
    a_vector.push_back(i);
  }

  int total = 0;
  a_iterator = a_vector.begin();
  while( a_iterator != a_vector.end() ) {
    total += *a_iterator;
    a_iterator++;
  }

you would see (in c#)

List<int> a_list = new List<int>();
for(int i=0; i < 100; i++)
{
  a_list.Add(i);
}
int total = 0;
foreach( int item in a_list)
{
  total += item;
}

Or more explicitly (without hiding the IEnumerator behind the foreach syntax sugar):

List<int> a_list = new List<int>();
for (int i = 0; i < 100; i++)
{
    a_list.Add(i);
}
int total = 0;
IEnumerator<int> a_enumerator = a_list.GetEnumerator();
while (a_enumerator.MoveNext())
{
    total += a_enumerator.Current;
}

As you can see, foreach just hides the .net enumerator for you.

So really, the ".net way" would be to simply allow people to create List< Local > items for themselves. If you do want to control iteration or make the collection a bit more custom, have your collection implement the IEnumerable< T > and/or ICollection< T > interfaces as well.

A near direct translation to c# would be pretty much what you assumed:

public class Native
{
  public class Local
  { 
     public string m_str;
     public int m_int;
  }

  private List<Local> m_Locals = new List<Local>();

  public List<Local> Locals
  {
    get{ return m_Locals;}
  }
}

Then a user would be able to

foreach( Local item in someNative.Locals)  
{
 ... 
}

Answer 2

@Phillip - Thanks, your answer really got me started in the right direction.

After seeing your code, and doing a little more reading in Nish's book C++/CLI in Action, I think using an indexed property that returns a const tracking handle to a Local instance on the managed heap is probably the best approach. I ended up implementing something similar to the following:

public ref class Managed
{
    public:
    ref class Local
    {
        String^ m_Str;
        int m_Int;
    };

    property const Local^ Locals[int]
    {
     const Local^ get(int Index)
     {
      // error checking here...
      return m_Locals[Index];
     }
    };

    private:
        List<Local^> m_Locals;
};