How to read elements in a VARIANT

[LindeA]

Hi,

I can not find out how to read what is in a VARIANT, which is the result of calling a method GetAllProperties. The doc of this method only specifies that a VARIANT is returned. It should contain some kind of strings. I tried this :

VARIANT names;
_variant_t v1;
VARTYPE vtype;
long ix;
...
x->GetAllProperties(&names);

if (V_ISARRAY(&names)) {
SafeArrayGetVartype(names.parray,&vtype);
// this returns 0x0800 in vtype, so I assume it is an array of _variant_t !!!
for (ix=0;ix<nprops;ix++) {
SafeArrayGetElement(names.parray,&ix,&v1);
_bstr_t s1(v1);
printf("name=%s\n",(char *)s1);
}
}

But this ends in an exception during the statement "_bstr_t s1(v1);"

What do I do wrong ???

Linde Ackermans

[BobS0327]

Maybe something like this, assuming it's a string..

_variant_t              vNetname;
char                   sNetname[40];

WideCharToMultiByte(CP_ACP,
			0,
			vNetname.bstrVal,
			-1,
			sNetname,
			sizeof(sNetname),
			NULL,
			NULL);

[Vanaj]

Hi,

I can not find out how to read what is in a VARIANT, which is the result of calling a method GetAllProperties. The doc of this method only specifies that a VARIANT is returned. It should contain some kind of strings. I tried this :

VARIANT names;
_variant_t v1;
VARTYPE vtype;
long ix;
...
x->GetAllProperties(&names);

if (V_ISARRAY(&names)) {
SafeArrayGetVartype(names.parray,&vtype);
// this returns 0x0800 in vtype, so I assume it is an array of _variant_t !!!
for (ix=0;ix<nprops;ix++) {
SafeArrayGetElement(names.parray,&ix,&v1);
_bstr_t s1(v1);
printf("name=%s\n",(char *)s1);
}
}


But this ends in an exception during the statement "_bstr_t s1(v1);"

What do I do wrong ???

Linde Ackermans

Please use code tags....


Try changing your printf() to this

printf("name=%s\n", (LPSTR)(_bstr_t) v1.bstrVal);

And delete this line

 _bstr_t s1(v1);

Also...check the return values on SafeArrayGetElement()

[Codeplug]

There are two issues with the code.

First, memory is being leaked through v1. SafeArrayGetElement() returns a copy of the BSTR and it's your responsibility to free it. You can either call v1.Clear() on each iteration or just use v1.GetAddress() which calls Clear() for you.

[NOTE: Stuff in red italics is incorrect!]

Second, operator char* on a _bstr_t does not give you a narrow (ascii) C-string. It's meant for byte-level access to the BSTR. You either treat it as a wide string or convert it as BobS0327 suggested.

Also keep in mind that if you do anything "wrong" with _variant_t or _bstr_t then a _com_error will be thrown. So you may want to wrap all the work with a try/catch.

    for (ix = 0; ix < nprops; ++ix) 
    {
        HRESULT hres = SafeArrayGetElement(V_ARRAY(names), &ix, v1.GetAddress());
        if (FAILED(hres))
            _com_issue_error(hres);

        _bstr_t s1(v1);
        //printf("name=%ls\n", (const wchar_t*)s1);
        wcout << L"name = " << (const wchar_t*)s1 << endl;
    }//for

>> (LPSTR)(_bstr_t)v1.bstrVal
Not good for reasons mentioned above.

>> And delete this line
That's fine if you know the returned variant will always be a BSTR. If it isn't, then the _bstr_t constructor will turn it into a string if it can (which is handy sometimes).

gg

[Vanaj]

>> (LPSTR)(_bstr_t)v1.bstrVal
Not good for reasons mentioned above.

gg

This exact is used to assign the bstrVal to a CString in 400,000 records with 20 fields in each record (8,000,000 times) each day for the last year within a try/catch and has not error'd out once...so I'm guessing the above code is good.

[Codeplug]

My mistake.

I was confusing _bstr_t with another class. It does in fact convert it to a narrow string using WideCharToMultiByte() with CP_ACP.

gg

[Vanaj]

My mistake.

I was confusing _bstr_t with another class. It does in fact convert it to a narrow string using WideCharToMultiByte() with CP_ACP.

gg

No problem...you will get much more respect from others here with this statement "My mistake" than you will with thousands of posts...

"To err is human; to forgive, divine." Alexander Pope