Easy way to manipulate BSTR using CString class

Introduction

When we do COM or ActiveX programming, we often have to cope with BSTR strings. Even thought there are APIs for BSTR manipulation, it is a bit difficult to safely manage the BSTR string. This article presents an easy way to manage the BSTR string using the CString class.

Managing BSTR is difficult

We usually use conversion macros like OLE2A, A2BSTR etc. But before using these macros, we should specify USES_CONVERSION macro at the beginning of the function in order to avoid compiler errors. We should also include the atlconv.h file to use these macros. It is also difficult to directly do operations like appending, searching etc. in a BSTR string.

Easy way

There is an easy way to manipulate BSTR string using the MFC CString class. CString constructor accepts the LPCWSTR. LPCWSTR is nothing but the unsigned short*. i.e. the BSTR. Apart from this the operator = is overloaded in CString class to support assignment operation. CString class also contains a member function AllocSysString() which returns the BSTR string corresponding to the CString object. Use the API SysFreeString to free the BSTR string when we no longer need the BSTR string.

Converting a BSTR to CString object sample:

void f(BSTR bStr)
{
    CString csStr = bStr; //or csStr(bStr);

    ...
    //Now we can do whatever we want with this csStr	

}

Converting a CString object to BSTR:

void SomeFunction()
{
    ...
    CString csStr = "Hello World!";

    BSTR bStr = csStr.AllocSysString();

    f(bStr) //maniplate the BSTR


    ::SysFreeString(bStr); //finished using the BSTR

    ...
}

Using temporary objects:

We can also use temporary objects of the CString class as follows. Suppose we want to compare the right 3 characters of a BSTR with another string, we can do as shown in the below function by using a temporary CString object.

void SomeFun()
{
...
    if(!CString(bStr).Right(3).Compare("man"))
    {

        //Do something

    }
...
}