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How do I check if CComBSTR starts with a specific prefix?

I come across a piece of code where I just need to do a prefix check for a given CComBSTR object (something like Y.StartsWith("X")

). C ++ is a bit foreign to me and my biggest problem is efficiency. I don't need to modify CComBSTR in any way. All I want is to return a boolean on whether it starts with an X prefix.

Looking at the operators listed in the API under CComBSTR Members , it allows very simple comparisons like ==,>, <, etc. I have two ideas on how I can try to solve this problem (see below). However, I don't have a deep understanding of what is the most efficient / easy way to do this. If I leave completely, let me know.

  • Use BSRTToArray to create an array where I then iterate over the first n indices to check if it has a specific prefix.
  • Get BSTR from CComBSTR and do some comparison in BSTR (haven't figured out how to do this yet).
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2 answers


wcsncmp

will compare a limited number of starting characters for you:



BOOL StartsWith(BSTR sValue, const WCHAR* pszSubValue)
{
    if(!sValue)
        return FALSE;
    return wcsncmp(sValue, pszSubValue, wcslen(pszSubValue)) == 0;
}
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   BOOL StartsWith(BSTR sValue, BSTR prefix)
         {
         if (!prefix)
            return TRUE;
         auto prefixlen = wcslen(prefix);
         if (prefixlen == 0)
            return TRUE;
         if (!sValue)
            return FALSE;
         return wcsncmp(sValue, prefix, prefixlen) == 0;
         }

   BOOL EndsWith(BSTR sValue, BSTR suffix)
         {
         if (!suffix)
            return TRUE; // always true if suffix is blank or null
         auto suffixlen = wcslen(suffix);
         if (suffixlen == 0)
            return TRUE; // always true if suffix is blank or null
         if (!sValue)
            return FALSE;
         auto valuelen = wcslen(sValue);
         if (suffixlen > valuelen)
            return FALSE;
         auto skip = valuelen - suffixlen;
         return wcsncmp(sValue + skip, suffix, wcslen(suffix)) == 0;
         }

I've included a suffixed version here, and included a variant of Roman R's answer that allows empty prefix strings.



Note: they are not tested well.

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