How to compare BSTR against a string in c/c++?

Question

wprintf(L"Selecting Audio Input Device: %s\n", 
                            varName.bstrVal);

if(0 == strcmp(varName.bstrVal, "IP Camera [JPEG/MJPEG]"))...

The above reports :

error C2664: 'strcmp' : cannot convert parameter 1 from 'BSTR' to 'const char *'

Answer 1

You have to use wcscmp instead:

if(0 == wcscmp(varName.bstrVal, L"IP Camera [JPEG/MJPEG]"))
{
}

Here is a description of the BSTR data type, it has a length prefix and a real string part which is just an array of WCHAR characters. It also has 2 NULL terminators.

The only thing to look out for is that the BSTR data type can contain embedded NULLs in the string portion, so wcscmp will only work in the cases where the BSTR does not contain embedded NULLs (which is probably most cases).

Answer 2

As a richer alternative to the C runtime, you could use the Unicode CompareString or CompareStringEx APIs in Win32. If you don't have charset issues to consider, wcscmp is fine though.

Answer 3

I always construct _bstr_t wrappers around BSTRs. It makes things quite a bit easier and more idiomatic:

if(std::string("IP Camera [JPEG/MJPEG]") ==
                   static_cast<const char*>( _bstr_t(varName.bstrVal) )
{
}

Answer 4

My solution:

static const std::wstring IPCamera = L"IP Camera [JPEG/MJPEG]";
if (varName.bstrVal == IPCamera {
  //...