COleVariant and strings

Hello,

I have a class which encapsulates a COleVariant. Here's a brief outlook of the class:

Code:

---

CEncaps::CEncaps() :
    m_pVariant(new COleVariant())
{
}

CEncaps::~CEncaps(void)
{
    if (m_pVariant)
    {
        delete m_pVariant;
        m_pVariant = NULL;
    }
}

BOOLEAN CEncaps::Read(CFile& file)
{
    BOOLEAN bOK = TRUE;

    switch (m_enumType)
    {
    case TYPE_BYTE:
        {
            m_pVariant->vt = VT_UI1;
            file.Read(&(m_pVariant->bVal), sizeof(BYTE));
        }
        break;
    // ...
    // All other numeric types are handled similarly to bytes
    // ...
    case TYPE_STRING:
        {
            char* pBuf = new char[m_nSize + 1];
            file.Read(pBuf, m_nSize);
            pBuf[m_nSize] = '\0';
            m_pVariant->SetString(pBuf, VT_BSTR);
        }
        break;
    }

    return bOK;
}

BOOLEAN CEncaps::Write(CFile& file)
{
    BOOLEAN bOK = TRUE;

    switch (m_enumType)
    {
    case TYPE_BYTE:
        {
            file.Write(&(m_pVariant->bVal), sizeof(BYTE));
        }
        break;
    // ...
    // All other numeric types are handled similarly to bytes
    // ...
    case TYPE_STRING:
        {
            BSTR bstr = m_pVariant->bstrVal;
            CString cstr(bstr);
            LPTSTR pBuf = cstr.GetBufferSetLength(m_nSize);
            file.Write(pBuf, m_nSize);
        }
        break;
    default:
        break;
    }

    return bOK;
}

---

(m_enumType and m_nSize are set by other methods not shown here for the sake of clarity.)

Now, my problem: this code works but produces memory leaks. I'm almost certain the leaks are caused only by strings. I find the BSTR type very confusing! I think I'm supposed to call SysFreeString() at some point, but my tries have been unsuccessful (produce error in the destructor).

So now I'm asking you code gurus :) I'd be really grateful if someone can help me correct this leak. Thanks in advance!

1. I wonder why do you create the COleVariant object of your class (m_pVariant) in the heap? :confused:
Creating it on the stack would work pretty good too!

2. I see at least one place where you create the char array in the heap and do NOT delete it at all! it is in CEncaps::Read method:

Quote:

---

Code:

---

    case TYPE_STRING:
        {
            char* pBuf = new char[m_nSize + 1];
            file.Read(pBuf, m_nSize);
            pBuf[m_nSize] = '\0';
            m_pVariant->SetString(pBuf, VT_BSTR);
        }
        break;

---

---

Why do you use char* type? What is wrong in using CString in this place?

Quote:

---

Originally Posted by VictorN

1. I wonder why do you create the COleVariant object of your class (m_pVariant) in the heap? :confused:
Creating it on the stack would work pretty good too!

---

Out of habit, I guess. I usually use pointers. I tried changing it to use the stack, but this produces an error box at runtime which simply says "Incorrect variable type." :/

Quote:

---

Originally Posted by VictorN

2. I see at least one place where you create the char array in the heap and do NOT delete it at all! it is in CEncaps::Read method:
Why do you use char* type? What is wrong in using CString in this place?

---

CFile::Read() expects a pointer to a buffer, that's why I used a char* buffer. Should I use the pointer returned by CString::GetBuffer()?

Also, I did not delete the array because I'm unsure of what COleVariant::SetString() does. Does it actually copy the contents of the buffer, or simply the pointer?

Thanks again for your help, VictornN.

Quote:

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Originally Posted by wlof

Out of habit, I guess. I usually use pointers. I tried changing it to use the stack, but this produces an error box at runtime which simply says "Incorrect variable type." :/

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It only means you are using it incorrect. Fix it!

Quote:

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CFile::Read() expects a pointer to a buffer, that's why I used a char* buffer. Should I use the pointer returned by CString::GetBuffer()?

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Yes! And then don't forget to call CString::ReleaseBuffer before using this CString object.

Quote:

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Also, I did not delete the array ...

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The why do you wonder you have memory leaks? :confused:

Quote:

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I'm unsure of what COleVariant::SetString() does. Does it actually copy the contents of the buffer, or simply the pointer?

---

What is wrong for you to read MSDN if you "unsure" yet? :confused:

Quote:

---

COleVariant::SetString
void SetString( LPCTSTR lpszSrc, VARTYPE vtSrc );

Parameters

lpszSrc
A null-terminated string to be copied into the new COleVariant object.

vtSrc
The VARTYPE for the new COleVariant object.
...

---

Quote:

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Originally Posted by VictorN

It only means you are using it incorrect. Fix it!

---

I fixed it. That was actually an unrelated bug, nothing to do with heap or stack.

Quote:

---

Originally Posted by VictorN

Yes! And then don't forget to call CString::ReleaseBuffer before using this CString object.

---

I've added a delete [] pBuf instruction to fix the leak, but I'll switch to CString anyway.

Quote:

---

Originally Posted by VictorN

The why do you wonder you have memory leaks? :confused:

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I didn't wonder I had memory leaks, I wondered at which point I was supposed to release the memory used to store the strings. If I understand correctly, VARIANT uses a BSTR to store strings, and I don't quite get how BSTR works...

Anyway, my bug is fixed. Thanks for your help!