C++ pass by reference WCHAR** into a function

Question

I have written following code where I am passing a WCHAR** reference to a function in order to get it filed inside the function. Inside the function, I am filling this 2 dimensional array. I am trying to use pass by reference to get this done.

BOOL get_file_names(WCHAR** outname)
{

    WCHAR **outhandlername = NULL;


    if (get_all_the_file_handelers( &outhandlername) > 0)
    {

        (*outname)[0] = *outhandlername[0];

    }

    return ret;
}

int get_all_the_file_handelers(WCHAR** outhandleName )
{



    for (i = 0; i < 10; i++)
    {
     WCHAR *handleName = get_handle_name(handle, i);

    outhandleName = (WCHAR**)realloc(outhandleName, sizeof(WCHAR)*(10 + 1));
    (*outhandleName) = (WCHAR*)realloc(*outhandleName, sizeof(WCHAR)*(1024));
    *outhandleName = handleName;

    }
    return 0;

}

But this does not seems to work, could anybody help me to understand how pass by reference works for array of WCHAR in a case like this. When I have WHCAR**, what is it correct to pass &WCHAR to second function, and inside the second function how should I be assigning the values for 2 dimensional WCHAR array

---

int _tmain(int argc, _TCHAR* argv[])
{
      WCHAR *outhandlename=NULL;
      get_file_names(&outhandlename);
      DBGLOG("handler name  %ls", outhandlename);

      return 0;
}

BOOL process_search_file_handle( WCHAR** str)
{
      *str = (WCHAR*) malloc(1024);
      *str = L"TestName";   
      return true;
}

BOOL get_file_names(WCHAR** outname)
    {

        WCHAR **outhandlername = NULL;


        if (get_all_the_file_handelers( &outhandlername) > 0)
        {

            *outname = outhandlername[0];
            DBGLOG("outhandlername value  %ls", outhandlername[0]);
            DBGLOG("str value  %ls", *outname);

        }

        return true;
    }

Answer 1

Once you set outHandleName to the output of realloc, you've already overwritten outHandleName, so you won't be able to change the value of outHandleName from the calling function.

You could say:

*outHandleName = (WCHAR**) realloc(...);

You also will have to change the method header for get_all_the_file_handelers to:

int get_all_the_file_handelers(WCHAR *** outHandleName)

This is because you're using a pointer to a double pointer in this method.

Also, you're not passing by reference - you're passing by pointer.

Additionally, you shouldn't be using realloc here, since you're doing an initial allocation for the array. Your array of 10 elements should be allocated before your loop, like so:

*outHandleName = (WCHAR **)malloc(sizeof(WCHAR *)*(10+1));
(*outHandleName)[10] = NULL;

Note that null assignment - you're allocating space for 11 items in an array of 10 items, so I assume you're using the last item as a placeholder - as NULL - to mark the end of the array.

Finally, you don't need to allocate space for a string buffer for each of the 10 handles, since you're getting a string back from get_handle_name.

As a bonus, you aren't returning anything, even though the rest of your code suggests you are.

The final method would be:

int get_all_the_file_handelers(WCHAR ***outHandleName) {
  *outHandleName = (WCHAR **)malloc(sizeof(WCHAR *)*11);
  (*outHandleName)[10] = NULL;
  for(int i = 0; i < 10; i++) {
    WCHAR *handleName = get_handle_name(handle, i);
    (*outHandleName)[i] = handleName;
  }
  return .... /* WHAT AM I RETURNING?? */ ;
}

Answer 2

Here is some example code that generates a list of wide strings using the standard library. You should learn how to deal with arrays of pointers later, but I suggest you learn how to use the STL first.

#include <cstdlib>
#include <experimental/filesystem>
#include <iostream>
#include <locale>
#include <string>
#include <vector>

#if _WIN32 || _WIN64
// Windows needs a little non-standard magic for this to work.
#include <io.h>
#include <fcntl.h>
#include <locale.h>
#endif

using std::endl;
using std::size_t;
using std::wcout;
using std::experimental::filesystem::path;

void init_locale(void)
// Does magic so that wcout can work.
{
#if _WIN32 || _WIN64
  // Windows needs a little non-standard magic.
  constexpr char cp_utf16le[] = ".1200";
  setlocale( LC_ALL, cp_utf16le );
  _setmode( _fileno(stdout), _O_U16TEXT );
#else
  // The correct locale name may vary by OS, e.g., "en_US.utf8".
  constexpr char locale_name[] = "";
  std::locale::global(std::locale(locale_name));
  std::wcout.imbue(std::locale());
#endif
}

std::vector<std::wstring> files_in_wd()
// Returns a list of filenames in the current working directory.
{
  const path cwd = std::experimental::filesystem::current_path();
  std::vector<std::wstring> filenames;

  for ( const path& file : cwd )
    filenames.emplace_back( file.filename().wstring() );

  return filenames;
}

int main(void)
{
  init_locale();

  const std::vector<std::wstring> filenames = files_in_wd();

  for ( const std::wstring& ws : filenames )
    wcout << ws << endl;

  return EXIT_SUCCESS;
}