HDevelop User's Manual

90 CHAPTER 4. CODE GENERATION
4.4 General Aspects of Code Generation
In the following, general differences in the behavior of a HDevelop program and its exported
versions are described.
4.4.1 Assignment
In HDevelop each time a new value is assigned to a variable its old contents are removed automatically,
independent of the type of the variable. In the exported code, this is also the case
for iconic objects (C++: ÀÓØ, Visual Basic: ÀÍÒØÝÔÇØ) and for the class ÀÌÙÔÐ
(C++) and the type ÎÖÒØ (Visual Basic), as they all have a destructor which removes the
stored data. As the language C does not provide destructors, the generated C code calls the
operators ÐÖ Ó and ×ØÖÓÝ ØÙÔÐ to remove the content of iconic output parameters
(ÀÓØ) and control output parameters (ÀØÙÔÐ) before each operator call.
However, problems arise if a tuple (variant) contains a handle, for example for a file, a window,
or for OCR. In this case, the memory of the handle is automatically removed but not the data
which it points at. In the exported programs, this data has therefore to be removed explicitely
by calling the corresponding operators ÐÓ× like ÐÓ× ÓÖ or ÐÓ× ÓÚ. Please insert
the ÐÓ× operators for all handles in use
¯ before a new value is assigned to a handle and
¯ at the end of the program.
In Visual Basic, the ideal way would be to use the specific COM classes for this kind of data in
combination with the member function. This exchange has to be done “by hand” because the
export is not able to generate appropriate code.
4.4.2 ÓÖ - Loops
HDevelop and the programming languages have different semantics for loops, which can cause
confusion. Because the problems are so rare and the generated code would become very difficult
to understand otherwise, the code generation ignores the different semantics. These differences
are:
1. In the programming languages, you can modify the loop variable (e.g., by setting it to
the end value of the condition) to terminate the loop. This can’t be done in HDevelop,
because here the current value is stored “inside” the ÓÖ-operator and is automatically
updated when it is executed again.
2. In the programming languages, you can modify the step range if you use a variable for
the increment. This is also not possible with HDevelop because the increment is stored
“inside” the ÓÖ-operator when the loop is entered.
3. The last difference concerns the value of the loop variable after exiting the loop. In the
programming languages, it has the value with which the condition becomes false for the
first time. In HDevelop it contains the end value, which was calculated when the loop
was entered.
HDevelop / 2000-11-16