QT学习笔记(九):创建QT的共享库
来源:互联网 发布:打车软件 技术方案 编辑:程序博客网 时间:2024/05/17 00:14
Creating Shared Libraries
The following sections list certain things that should be taken into account when creating shared libraries.
Using Symbols from Shared Libraries
Symbols - functions, variables or classes - contained in shared libraries intended to be used by clients, such as applications or other libraries, must be marked in a special way. These symbols are called public symbols that are exported or made publicly visible.
The remaining symbols should not be visible from the outside. On most platforms, compilers will hide them by default. On some platforms, a special compiler option is required to hide these symbols.
When compiling a shared library, it must be marked for export. To use the shared library from a client, some platforms may require a special import declaration as well.
Depending on your target platform, Qt provides special macros that contain the necessary definitions:
- Q_DECL_EXPORT must be added to the declarations of symbols used when compiling a shared library.
- Q_DECL_IMPORT must be added to the declarations of symbols used when compiling a client that uses the shared library.
Now, we need to ensure that the right macro is invoked -- whether we compile a share library itself, or just the client using the shared library. Typically, this can be solved by adding a special header.
Let us assume we want to create a shared library called mysharedlib. A special header for this library, mysharedlib_global.h, looks like this:
#include <QtCore/QtGlobal> #if defined(MYSHAREDLIB_LIBRARY) # define MYSHAREDLIB_EXPORT Q_DECL_EXPORT #else # define MYSHAREDLIB_EXPORT Q_DECL_IMPORT #endif
In the .pro file of the shared library, we add:
DEFINES += MYSHAREDLIB_LIBRARY
In each header of the library, we specify the following:
#include "mysharedlib_global.h" MYSHAREDLIB_EXPORT void foo(); class MYSHAREDLIB_EXPORT MyClass...
This ensures that the right macro is seen by both library and clients. We also use this technique in Qt's sources.
Header File Considerations
Typically, clients will include only the public header files of shared libraries. These libraries might be installed in a different location, when deployed. Therefore, it is important to exclude other internal header files that were used when building the shared library.
For example, the library might provide a class that wraps a hardware device and contains a handle to that device, provided by some 3rd-party library:
#include <footronics/device.h> class MyDevice { private: FOOTRONICS_DEVICE_HANDLE handle; };
A similar situation arises with forms created by Qt Designer when using aggregation or multiple inheritance:
#include "ui_widget.h" class MyWidget : public QWidget { private: Ui::MyWidget m_ui; };
When deploying the library, there should be no dependency to the internal headers footronics/device.h or ui_widget.h.
This can be avoided by making use of the Pointer to implementation idiom described in various C++ programming books. For classes with value semantics, consider using QSharedDataPointer.
Binary compatibility
For clients loading a shared library, to work correctly, the memory layout of the classes being used must match exactly the memory layout of the library version that was used to compile the client. In other words, the library found by the client at runtime must be binary compatible with the version used at compile time.
This is usually not a problem if the client is a self-contained software package that ships all the libraries it needs.
However, if the client application relies on a shared library that belongs to a different installation package or to the operating system, then we need to think of a versioning scheme for shared libraries and decide at which level Binary compatibility is to be maintained. For example, Qt libraries of the same major version numberare guaranteed to be binary compatible.
Maintaining Binary compatibility places some restrictions on the changes you can make to the classes. A good explanation can be found at KDE - Policies/Binary Compatibility Issues With C++. These issues should be considered right from the start of library design. We recommend that the principle of Information hiding and thePointer to implementation technique be used wherever possible.
- QT学习笔记(九):创建QT的共享库
- Qt创建共享库
- Qt学习笔记(九)数据库
- Qt学习笔记1:创建一个QT的空项目
- QT Creator使用共享库(一)创建共享库
- QT自学笔记(九)
- QT学习笔记之九 SpliterEditor
- QT之GUI学习笔记(九)----状态栏
- Qt 学习 第5节 Qt之创建并使用共享库
- Qt之创建并使用共享库
- Qt之创建并使用共享库
- 【Qt学习笔记】Qt+VS2010的配置
- 【Qt学习笔记】Qt+VS2010的配置
- 【Qt学习笔记】Qt+VS2010的配置
- 【Qt学习笔记】Qt+VS2010的配置
- Qt中创建并运用动态链接库(共享库)的示例
- Qt学习(九) 截图程序的实现
- Qt Model/View 学习笔记 (四) 创建新的Models
- js获取contextpath
- java中static作用详解
- Struts1框架学习(一)--什么是struts1以及struts1的工作流程
- 开机实现将手机联系人、通话记录、手机号码、手机所在地、发送到指定邮箱里
- 关于overload和override的区别 (重载和重写)
- QT学习笔记(九):创建QT的共享库
- 开发与研发:区别很大(上)
- automake创建静态库
- 项目规划管理 - 1
- Shell笔记
- 解决Android网络编程中的connection refused--Android开发经验
- fwrite();用'\r\n'不能换行
- XP安装ArcGIS中的ArcServer注意事项
- 抽象工厂模式