Monday, May 09, 2005

12. What is the layout of modifiable descriptors?

Modifiable descriptors all derive from the base class TDes, which itself is a subclass of TDesC. TDes has an additional 4-byte member variable, which is used to store the maximum length of data currently allocated to the descriptor.

TDes defines a range of methods for modifying string data. Like the non-modifiable descriptors, all the manipulation is inherited by the derived classes, and works regardless of their type. The subclasses only implement specific methods for construction and copy assignment.

Apart from the allocation methods of the heap buffer descriptors, no descriptor functions allocate any memory. So any modification which extends the data of the descriptor, eg Append(), will check that there is sufficient memory available before proceeding. The contents of the descriptor can shrink and expand at will, as long as the length does not exceed the maximum. If the length of the descriptor contents is less than the maximum length, the end of the block of memory allocated to the descriptor is simply unused.

Descriptor code uses assertion statements to ensure that the maximum length of the descriptor is sufficient, before proceeding. The checks are made in both debug and release builds, and panic if an overflow would occur. This avoids the hard-to-trace memory scribbles and buffer overflows typical of C style strings.

(See also 11. What is the layout of non-modifiable descriptors?)

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