How to pass an array of strings through a COM (C++) API

Question

We have a COM API for our application (which is written in VC++) which exposes a few functionalities so that the users can automate their tasks. Now, I'm required to add a new method in that, which should return a list/array/vector of strings. Since I'm new to COM, I was looking at the existing methods in the .idl file for that interface.

One of the existing methods in that idl file looks like this:

interface ITestApp : IDispatch
{
    //other methods ..
    //...
    //...
    //...
    [id(110), helpstring("method GetFileName")] HRESULT GetFileName([out, retval] BSTR *pFileName);
    //...
    //...
    //...
};

My task is to write a similar new method, but instead of returning one BSTR string, it should return a list/array/vector of them.

How can I do that?

Thanks!

Answer 1

Since yours is an automation-compatible interface, you need to use safearrays. Would go something like this:

// IDL definition
[id(42)]
HRESULT GetNames([out, retval] SAFEARRAY(BSTR)* names);

// C++ implementation
STDMETHODIMP MyCOMObject::GetNames(SAFEARRAY** names) {
  if (!names) return E_POINTER;
  SAFEARRAY* psa = SafeArrayCreateVector(VT_BSTR, 0, 2);

  BSTR* content = NULL;
  SafeArrayAccessData(psa, (void**)&content);
  content[0] = SysAllocString(L"hello");
  content[1] = SysAllocString(L"world");
  SafeArrayUnaccessData(psa);

  *names = psa;
  return S_OK;
}

Error handling is left as an exercise for the reader.