Pointer to a pointer C++/CLI wrapper

A nicety of managed code is that it uses a garbage collector and has strongly typed arrays. So returning an array as a function return value is well supported. Which makes your ideal wrapper function something like this:

  array<Byte>^ Wrapper::Decode(array<Byte>^ encodedBuffer) {
      pin_ptr<Byte> encodedPtr = &encodedBuffer[0];
      Byte* decoded = nullptr;
      unsigned int decodedLength
      int err = unmanagedDecoder->Decode(encodedPtr, encodedBuffer->Length, &decoded, &decodeLength);
      // Test err, throw an exception
      //...
      array<Byte>^ retval = gcnew array<Byte>(decodedLength);
      Marshal::Copy((IntPtr)decoded, retval, 0, decodedLength);
      free(decoded);   // WATCH OUT!!!
      return retval;
  }

Do note the // WATCH OUT comment. You need to destroy the buffer that the decoder allocated. Doing this correctly requires knowing how the decoder manages its memory and usually makes it very important that your C++/CLI code shares the same CRT as the decoder module. This tends to go wrong badly if there is no good protocol or you can't compile the decoder source code.