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<concept id="GUID-260C7ACD-0024-59DE-B1C6-0C852E44C457" xml:lang="en"><title>Dynamically Loading Link Libraries Overview</title><shortdesc>Description of Polymorphic DLLs.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-25D3E357-BE6E-40BD-897D-DA5EB92029C5"><title>Purpose</title> <p>Dynamically load libraries during program execution.</p> <p>These
interfaces are used by programs that need to load libraries at run-time,
that have a common interface, but different concrete behavior. Because
of this relationship between interface and behaviour, such libraries
are called <i>polymorphic DLLs</i>. </p> </section>
<section id="GUID-5444CFD3-5FC2-4160-BD8A-8BD7154402B3"><title>Architectural
relationships</title> <p>Many parts of the system use polymorphic
DLLs. Communications components, such as Sockets and Messaging, use
architectures in which particular communications protocols are provided
as polymorphic DLLs, and are loaded as needed. UI application programs
are also polymorphic DLLs that are loaded when the user starts the
program.</p> </section>
<section id="GUID-2A379537-7B05-445F-83FC-C4396794A3CD"><title>Description</title> <p>The API has three key concepts: polymorphic DLL, polymorphic
DLL handle, and polymorphic DLL function pointer. </p> </section>
<section id="GUID-0A0DF81E-94C0-4D91-8F2A-1DFBE542E019"><title>Polymorphic
DLL</title> <p>A program that uses a type of polymorphic DLLs defines
their interface in terms of a single abstract class whose functions
are declared as pure virtual. Each type of polymorphic DLL is given
a unique identifier (UID). </p> <p>A particular concrete DLL:</p> <ul>
<li id="GUID-E073605D-1AC4-55B9-94AA-FD1A761B5448"><p>implements the
interface by defining and implementing a concrete class derived from
the abstract class.</p> </li>
<li id="GUID-AF95C27B-841D-53B5-836B-A55B4ED3E5F3"><p>exports a function
that creates an object of the derived class type. All other functions
in the DLL are virtual and called through the base class interface.</p> </li>
</ul></section>
<section id="GUID-825B7916-7E13-42B5-BAE4-A7009679E4F2"><title>Polymorphic
DLL handle</title><p>The polymorphic DLL handle allows a program to
load and close a particular polymorphic DLL. It also allows the caller
to obtain pointers to functions exported by the DLL. The system can
check that a polymorphic DLL is of the correct type by checking the
type UID value.</p> <p>The polymorphic DLL handle interface is provided
by <xref href="GUID-25327159-83D6-3507-B187-09EA4BB3727F.dita"><apiname>RLibrary</apiname></xref>.</p> <p>A wildcard file system search
for polymorphic DLLs can be done through <xref href="GUID-1B05E2E3-B64E-36A8-BFFF-430028113BA4.dita"><apiname>TFindLibrary</apiname></xref>.</p> </section>
<section id="GUID-C16BECEC-B59A-4FFE-94F0-548DB602A0BA"><title>Polymorphic
DLL function pointer</title> <p>An exported function is called through
a pointer to function obtained through a polymorphic DLL handle.</p> <p>The polymorphic DLL function pointer type is provided by the <xref href="GUID-7E0A7BA7-0EF8-3832-A1EA-B85FFADD1A9B.dita"><apiname>TLibraryFunction</apiname></xref> typedef. The caller casts the pointer
to the real type of the exported function before using it.</p> </section>
</conbody><related-links>
<link href="GUID-AB705D21-87A3-5F32-A4B9-17B7593DD5C3.dita"><linktext>UID
Manipulation Overview</linktext></link>
</related-links></concept>