Difference between revisions of "Development/Tutorials/Creating Libraries"

Revision as of 17:39, 7 April 2008

Development/Tutorials/Creating Libraries

Abstract

If a part of your code could be used by more than one software module, like other programs or plugins, you should put that part in a shared library. This tutorial tells how to add the library to the buildsystem and how to prepare the source code.

Separating the code

It is good practise to put all source files for a library in a separate directory.

This tutorial assumes you want to create a shared library named myshare, which contains the classes MyAClass and MyBClass. Both classes use internally the classes MyInternalCClass and MyInternalDClass, but these are not found in their public interfaces.

The classes are declared and defined in the files myaclass.h, myaclass.cpp, mybclass.h, mybclass.cpp, myinternalcclass.h, myinternalcclass.cpp, myinternaldclass.h, and myinternaldclass.cpp.

Adding the library to the buildsystem

You also need a file CMakeLists.txt in the same directory as the source files: set( myshared_LIB_SRCS

 myaclass.cpp
 mybclass.cpp
 myinternalcclass.cpp
 myinternaldclass.cpp

 ${KDE4_KDECORE_LIBS}

 PROPERTIES VERSION ${GENERIC_LIB_VERSION} SOVERSION ${GENERIC_LIB_SOVERSION}

install( TARGETS myshared ${INSTALL_TARGETS_DEFAULT_ARGS} )

The instructions are similar to the ones for a program. But instead of calling kde4_add_executable() you use kde4_add_library() to register the library myshared. The parameter SHARED is needed as this declares the library is a shared one, not a static.

Like with a program, other libraries that are used have to be defined with target_link_libraries().

The version number of the library is set by set_target_properties().

And install() moves the library to a place where it can be loaded.

Declaring exported classes and methods

If a library gets compiled, all functions or class methods it offers get listed as so called symbols in a table. Each symbol points to the offset in the library where the corresponding code can be found.

Adding also functions or class methods to the table which are not intended to be called from the outside is of no use. It just bloats the table and asks others to make calls to them against your will. For the library myshared you do not want to add the methods of the classes MyInternalCClass and MyInternalDClass.

In the KDE buildsystem you need to put explicitly a tag to all the classes and functions which the library should provide externally. This tag has to have a different name for each library.

For this add a file myshared_export.h to the directory of the library, containing:

  // We are building this library

  // We are using this library

This macro code defines the tag MYSHARED_EXPORT for the library.

Now you prepare all classes by adding a include for the header with the library's export tag and putting the export tag defined above in front of the class name, like this:

class MYSHARED_EXPORT MyAClass { // class declaration as usual };

And the same with the other class:

class MYSHARED_EXPORT MyBClass { // class declaration as usual };

 

Warning
Beware that header-only classes, even additionally based on templates, must not have an export tag. Their methods do not end as any symbol in the library. Instead all machine code is created on demand in the external calling software, if compiled.

Enabling others to compile using your library

You need also tell the buildsystem to install the header files. Without this external code can not build, as it can not include the files and thus the class and methods declarations of your library. You do this by adding to the library's CMakeLists.txt:

set( myshared_LIB_HDRS

 myshared_export.h
 myaclass.h
 mybclass.h

 DESTINATION ${INCLUDE_INSTALL_DIR}/myshared
 COMPONENT Devel

)

Of course also myshared_export.h is installed, as it gets included by the headers of both classes.

External KDE code which should use the library's classes and functions now can include the headers by the lines:

 

Tip

By installing the header files you are releasing your library officially. External code will rely on the interfaces declared by the headers. So you need to care for binary and behaviour compatibility if developing your library further. Any changes in the next release should be reflected in the proper part of the version number, so external code gets compiled and linked against the matching version.