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DynaPDF Manual - Page 22

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Custom library changes
Page 22 of 750
In most cases both object types are defined as normal classes, which contain their own
constructors and destructors to initialize and destroy allocated memory.
Global objects can be deleted or marked as deleted at runtime. Resource objects must never be
deleted if the object was already used.
General design requirements
We describe here only the general rules which must be taken into account when extending
DynaPDF with certain features. We do not explain how the entire library works; this would fill an
entire book. To understand how an object must be written to the file we recommended that you
debug especially the function CloseFile(). All objects must be prepared for writing in a two stage
phase to reserve object numbers. The first stage assignes object numbers to all objects and the
second run writes all objects to file. Objects must be written in the exact order in which they were
previously prepared for writing.
A PDF file is described in memory as large set of classes which can be referenced or used multiple
times by other classes. Due to the references which are stored in certain classes we must define a
strict set of rules so that no exception occurs if an object must be deleted:
1. The owner of all resources and global objects is CPDF. No other class is permitted to
destroy an object class or change its values.
2. All object classes must be derived from CBaseObject. This class holds the object number as
well as several flags to determine whether the object was used, created, or already written,
and whether it is part of the first page. This class contains the function CreateObject()
which must be called in CPDF::PreparePageObjects() if the object is part of a page. If the
object is not included in a page object then CreateObject() must be called in
CPDF::PrepareObjects().
3. All classes which hold pointers to other object classes must check the "Used" flag before
writing the object data to the file (GetUsed() is a member of CBaseObject and returns true
if the used flag was set).
4. All classes must be well initialized so that the class can be deleted at any time without
causing memory leaks or other unwanted side effects.
5. No object class is permitted to unset the "Used" flag of other object classes.
6. Page resources such as fonts, images, templates and so on must NEVER be deleted at
runtime and their "Used" flag must NEVER be unset.
7. All resource classes must be derived from CBaseResource.
8. Object classes must set the "Used" flag of the resource object, if the class is used by this
object.
 

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