Question

In the CString header file (be it Microsoft’s or Open Foundation Classes – http://www.koders.com/cpp/fid035C2F57DD64DBF54840B7C00EA7105DFDAA0EBD.aspx#L77 ), there is the following code snippet

struct CStringData
{   
    long nRefs;
    int nDataLength;
    int nAllocLength;
    TCHAR* data() { return (TCHAR*)(&this[1]); };
    ...
};

What does the (TCHAR*)(&this[1]) indicate?

The CStringData struct is used in the CString class (http :// http://www.koders.com/cpp/fid100CC41B9D5E1056ED98FA36228968320362C4C1.aspx).

Any help is appreciated.

Answer 1

CString has lots of internal tricks which make it look like a normal string when passed e.g. to printf functions, despite actually being a class – without having to cast it to LPCTSTR in the argument list, e.g., in the case of varargs (...) in e.g. a printf. Thus trying to understand a single individual trick or function in the CString implementation is bad news. (The data function is an internal function which gets the ‘real’ buffer associated with the string.)

There’s a book, MFC Internals that goes into it, and IIRC the Blaszczak book might touch it.

EDIT: As for what the expression actually translates to in terms of raw C++:-

TCHAR* data() { return (TCHAR*)(&this[1]); };

this says “pretend you’re actually the first entry in an array of items allocated together. Now, the second item isnt actually a CString, it’s a normal NUL terminated buffer of either Unicode or normal characters – i.e., an LPTSTR”.

Another way of expressing the same thing is:

TCHAR* data() { return (TCHAR*)(this + 1); };

When you add 1 to a pointer to T, you actually add 1* sizeof T in terms of a raw memory address. So if one has a CString located at 0x00000010 with sizeof(CString) = 4, data will return a pointer to a NUL terminated array of chars buffer starting at 0x00000014

But just understanding this one thing out of context isnt necessarily a good idea.

Why do you need to know?