Sub-Strings – New Design

Previously with the original String class, as an optimization, sub-string functions were handled differently than the other string functions.  Instead of returning a temporary string, they simply adjusted what part of the string they referred to, and would therefore work for either temporary strings (results of other string functions or operators) or reference strings (from variables).  Sub-strings assignments would be handled with an assign sub-string code that would work with the result of a sub-string of a variable reference.  This was detailed by the series of posts in May, 2010.

With the change to the QString class, this optimization will not work, and is not necessary.  The sub-string functions (LEFT$, MID$, and RIGHT$) will work like the other string functions and operators where they will return a temporary string.  However, sub-string assignments will need to be handled differently.  There will need to be specific new codes for handling sub-string assignments, to be named AssignLeft, AssignMid2, AssignMid3, and AssignRight.

Consider the following sub-string assign along with the old translation and the proposed new translation:

LEFT$(A$,5)=B$
Old: A$<ref> 5 LEFT$(<ref> B$ AssignSub$
New: A$<ref> 5 B$ AssignLeft

With the old translation, the sub-string reference would be on the stack (along with the value to assign) for the generic AssignSub$ to process.  The new translation is simpler where the new AssignLeft code will expect a regular variable reference, the length argument of the LEFT$ function, and the value to assign.  Internally all the sub-string assignment codes will use the QString::replace() function.