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What is a (void**)?

Discuss What is a (void**)? in the C Programming forum on Dev Shed.
What is a (void**)? C programming forum discussing all C derivatives, including C#, C++, Object-C, and even plain old vanilla C. These languages are low level languages, and used on projects such as device drivers, compilers, and even whole computer operating systems.

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  #1  
Old March 22nd, 2002, 05:37 PM
MJEggertson MJEggertson is offline
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What is a (void**)?
The title says pretty much everything I want to know. I'm trying to wrap my brain around COM, and I've never seen a cast like (void**) before. It appears in the QueryInterface method of COM objects:

HRESULT QueryInterface( REFIID iid, void** ppvObject );

I know what a void pointer is, I guess I'm just confused with the ** part. Is it just a pointer to a pointer?
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Old March 23rd, 2002, 09:46 AM
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I don't know anything about this type of programming but I believe you are correct when you say it is a pointer to a pointer. The following link states:

ppvObject - An [out] pointer to the interface pointer identified by iid.

http://www.isd.mel.nist.gov/project...n/IUnknown.html
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Old March 23rd, 2002, 09:52 AM
M.Hirsch M.Hirsch is offline
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imagine a list of strings that you want to pass to a function.

each string is a char*, a list is passed via its address, so the "list of strings" is a char**

the void** is just more general.
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Old March 23rd, 2002, 04:04 PM
MJEggertson MJEggertson is offline
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Thanks. Its good to be able to confirm things. I hate "going on faith," for things like this, because eventually, assuming things catches up with you-- especially if your assumption was wrong.

Now if I could just figure out how to use these darn COM interfaces... . I didn't want to learn the COM, but the project I've started pretty much demands it if I'm gonna see it to completion. I guess I'm off to the bookstore this weekend.

It was supposed to be one of those 'simple' project. *cough* The amount of times I've heard that...
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Old March 23rd, 2002, 07:44 PM
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Ok, I'm out of ideas I guess. I'm hoping someone can offer some advice. Its a pretty specific problem. I have a view derived from CHTMLView. Its pretty basic, I use a template to put an 'empty' (html) document in the view, then I'm supposed to browse the DOM and populate/retrieve the form elements with data from the actual application's document (ie: the doc data from the doc/view architecture).

Here's the catch though, I'm able to browse the dom, but I can't for the life of me figure out how to access/modify any of its attributes. Here's what I got so far.

Code:
// Get the document's IDispatch pointer.
IDispatch* pHtmlDoc = GetHtmlDocument();

// Obtain a pointer to the IHTMLDocument2 interface.
IHTMLDocument2* pIDocument = NULL;
HRESULT hr;
hr = pHtmlDoc->QueryInterface(IID_IHTMLDocument2, (void**) &pIDocument);
if (SUCCEEDED(hr))
{

    // Obtain a pointer to the HTMLElements interface.
    IHTMLElementCollection* pIElements = NULL;
    hr = pIDocument->get_all(&pIElements);
    if (SUCCEEDED(hr))
    {

        // Obtain a pointer to all the Input elements from the collection
        IHTMLElementCollection* pInputs = NULL;
        hr = pIElements->tags(_variant_t("input"), (IDispatch**) &pInputs);
        if (SUCCEEDED(hr))
        {

            // For now, try to get element 0.
            // Eventually will need to try to iterate over all elements.
            IHTMLInputTextElement* pIThisElement = NULL;
            hr = pInputs->item(_variant_t((long) 0), _variant_t((long) 0),
                (IDispatch**) &pIThisElement);
            if (SUCCEEDED(hr))
            {

                // How do I get element name/value?
                pIThisElement->Release();
            }
            else
                ErrorMessage(_T("Failed at pIThisElement"));
        
            pInputs->Release();
        }
        else
            ErrorMessage(_T("Failed at pInputs"));

        pIElements->Release();
    }
    else
        ErrorMessage(_T("Failed at pIForms"));

    pIDocument->Release();
}
else
    ErrorMessage(_T("Failed at pIDocument"));

pHtmlDoc->Release();


The ErrorMessage() function is just a little shortcut to a message box to tell me exactly where the browsing failed.

How do I use the pIThisElement interface to edit (if the app is in a load procedure), or read (if the app is in a save procedure)? I can't seem to figure out the pIThisElement->get_value() or similar functions.
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Old March 25th, 2002, 05:42 PM
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Have you tried this:

Code:
CComBSTR bstr;

...
....
pIThisElement->get_name(&bstr); // To get the name
CString strTag = bstr;

pIThisElement->get_value(&bstr); // To get the value
pIThisElement->set_value(bstr); // To set the value
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Old March 26th, 2002, 01:38 AM
MJEggertson MJEggertson is offline
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Unfortunately, yes. I get an assertion error.


File: i386\chkesp.c
Line: 42

The value of ESP was not properly saved across a function call. This is usually a result of calling a function declared with one calling convention with a function pointer declared with a different calling convention.


Funny thing is, I get this error even if I pass a plain old &BSTR to the get/put functions. Its got me stumped.
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