Understanding Strings In COM

OLECHARs are the standard way to create strings in COM code and by far the most comfortable as long as C and C++ are used in both the client side and the server side. Other languages and tools bring their burden of special constraints that open the way to another kind of string, which constitute the topic of the next paragraph.

BSTRs

B-strings, more properly called Basic strings, are a special kind of string format. Instead of comprising a classic array of characters followed by a NUL character (code \0) that marks the termination of the array, the structure of the data in memory is a superset of OLECHAR. In short, a BSTR is a null-terminated array of OLECHARs prefixed by its length. The string length is determined by the character count, not by the index of the first null character.

This presence of the length of the object before the actual array data renders these strings suitable for manipulation in high-level tools like Visual Basic (for which this string format was invented in the first place) and Java on a COM-aware virtual machine like Microsoft's JVM. Actually, there is no other way to exchange string-like data with components written in those languages than to employ BSTRs. While in C and C++ the developer has to understand and use the data type in a rather uncomfortable manner, both Visual Basic and Java encapsulate them into their traditional string types, respectively String and java.lang.String. The final developer is therefore shielded from the subtleties of the organization of the raw bytes in memory. Moreover, the tools take care of allocating and freeing the memory required to contain their content without the programmer needing to know how this process works behind the scenes.