CODEWITHSUNDEEP
c++dlllpwstr
Elduckle
Elduckle

Reputation: 23

How to set a LPWSTR pointer which has been provided by reference to a function?

I am replacing an old DLL which has a function, passing an LPWSTR by reference. The functions should create a new wchar buffer and return the newly created LPWSTR pointer. I found a solution, but I think it could be solved better!?

By the main proramm the DLL function is called like this:

LPWSTR data = null;
funcA(&data);

The function shoud creat a new wchar buffer and return the address.

So here is what I do (it works):

void funcA(LPWSTR szData)
{
  LPWSTR newData = L"DEMODATA";
  LPWSTR newBuffer = new TCHAR[9];
  wmemcpy(newBuffer, newData, 9);

  memcpy(szData, (LPWSTR)&newBuffer , sizeof(LPWSTR));  
}

Is it possibe to write the last line better readable? I tried to assign the new pointer but this does not work:

szData = (LPWSTR)&newBuffer; // not working!

Upvotes: 2

Views: 1246

Answers (3)

Aconcagua
Aconcagua

Reputation: 25526

passing an LPWSTR by reference

But you don't do so...

void funcA(wchar_t* data); // LPWSTR resolved to what it actually is
                           // using correct pointers illustrates better
                           // than Microsoft's (valueless?) pointer typedefs...

The original pointer you pass is copied into the function argument, assignment then occurs to this copy.

You actually need to be able to assign to outside address, so you need to do what you already described:

void funcA(wchar_t*& data); // reference to pointer
//                 ^ (!)
{
    wchar_t const* newData = L"DEMODATA"; // literals are const!
    data = new wchar_t[9]; // you can assign directly (especially: no need for memcpy)
    wmemcpy(data, newData, 9);
}

Just for further illustration, might help to better understand: C style pointer to pointer:

void funcA(wchar_t** data); // pointer to pointer
//                 ^
{
    wchar_t* newData = new wchar_t[9];
    wmemcpy(newData, L"DEMODATA", 9);
    *data = newData;
//  ^ (!)
};

Usage of this variant:

wchar_t* text;
funcA(&text);
//    ^ (!)

Upvotes: 2

Tiberiu Maran
Tiberiu Maran

Reputation: 2173

memcpy(szData, (LPWSTR)&newBuffer , sizeof(LPWSTR));

is equivalent to

*(LPWSTR*)szData = newBuffer; //this copies into the address pointed to by szData

not to

szData = (LPWSTR)&newBuffer; // this copies into szData

Upvotes: 0

Bartek Banachewicz
Bartek Banachewicz

Reputation: 39370

The function shoud creat a new wchar buffer and return the address.

Then do just that:

LPWSTR funcA() {
  LPWSTR newData = L"DEMODATA";
  LPWSTR newBuffer = new TCHAR[9];
  wmemcpy(newBuffer, newData, 9);

  return newBuffer;
}

LPWSTR data = funcA();

That being said, consider using unique_ptr to store owned pointers. This will work, but it's bad practice. Even better, use std::string to process data and only convert to WinAPI pointers when necessary.

Upvotes: 1

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