ODB：C++ ORM框架详解与使用指南

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"ODB操作手册" ODB（Object Database Bridge）是一个强大的开源C++对象关系映射（ORM）框架，它能够使开发人员在多种平台上轻松地处理数据库操作。ORM框架允许开发者使用面向对象的编程方式来操作数据库，而无需直接编写SQL语句。ODB支持包括MySQL、SQLite、PostgreSQL和Oracle在内的多种主流数据库系统。 ODB的设计目的是简化C++应用程序与数据库之间的交互，通过自动持久化C++对象到数据库表，实现了数据层和业务逻辑层的解耦。在ODB中，C++类可以直接映射到数据库表，类的成员变量对应表中的列，对象实例则对应表的行。手册中的"1.1 Architecture and Workflow"部分详细介绍了ODB的工作原理和流程。ODB的架构基于C++模板元编程，可以在编译时生成相应的数据库访问代码，确保了性能和类型安全。工作流程通常包括定义持久化类、生成数据库支持代码、编译并运行应用程序。 "1.2 Benefits"章节阐述了使用ODB带来的好处，比如减少手写SQL代码，提高开发效率，提供强类型安全，以及能够更好地适应C++的面向对象特性，如继承和多态。此外，ODB还支持事务、查询优化、对象缓存等功能，增强了应用程序的灵活性和可维护性。 "2 Hello World Example"是入门教程，引导读者快速了解如何使用ODB。首先，"2.1 Declaring a Persistent Class"部分讲解了如何声明一个持久化类，通过在类定义中使用特殊的注释或宏来指定数据库映射。接着，"2.2 Generating Database Support Code"介绍了如何利用ODB的工具自动生成与数据库交互的代码。最后，"2.3 Compiling and Running"指导用户如何编译和运行程序，实现对数据库的基本操作，如插入、查询和更新对象。在"2.4 Making Objects Persistent"中，会进一步解释如何将C++对象保存到数据库，以及如何从数据库中加载和检索对象。这部分内容通常涵盖对象的创建、持久化、查询和更新操作，这些都是ORM框架的核心功能。 ODB的文档还涵盖了高级主题，如复杂查询、存储过程、自定义类型映射、性能调优以及与其他C++库和框架的集成。对于任何需要在C++项目中进行数据库操作的开发者来说，ODB都是一个强大且灵活的工具，其详尽的手册提供了全面的指导和支持。

1.2 Benefits

The traditional way of saving C++ objects to relational databases requires that you manually

write code which converts between the database and C++ representations of each persistent class.

The actions that such code usually performs include conversion between C++ values and strings

or database types, preparation and execution of SQL queries, as well as handling the result sets.

Writing this code manually has the following drawbacks:

Difficult and time consuming. Writing database conversion code for any non-trivial appli-

cation requires extensive knowledge of the specific database system and its APIs. It can also

take a considerable amount of time to write and maintain. Supporting multi-threaded appli-

cations can complicate this task even further.

Suboptimal performance. Optimal conversion often requires writing large amounts of extra

code, such as parameter binding for prepared statements and caching of connections, state-

ments, and buffers. Writing code like this in an ad-hoc manner is often too difficult and time

consuming.

Database vendor lock-in. The conversion code is written for a specific database which

makes it hard to switch to another database vendor.

Lack of type safety. It is easy to misspell column names or pass incompatible values in

SQL queries. Such errors will only be detected at runtime.

Complicates the application. The database conversion code often ends up interspersed

throughout the application making it hard to debug, change, and maintain.

In contrast, using ODB for C++ object persistence has the following benefits:

Ease of use. ODB automatically generates database conversion code from your C++ class

declarations and allows you to manipulate persistent objects using simple and thread-safe

object-oriented database APIs.

Concise code. With ODB hiding the details of the underlying database, the application logic

is written using the natural object vocabulary instead of tables, columns and SQL. The

resulting code is simpler and thus easier to read and understand.

Optimal performance. ODB has been designed for high performance and low memory

overhead. All the available optimization techniques, such as prepared statements and exten-

sive connection, statement, and buffer caching, are used to provide the most efficient imple-

mentation for each database operation.

Database portability. Because the database conversion code is automatically generated, it is

easy to switch from one database vendor to another. In fact, it is possible to test your appli-

cation on several database systems before making a choice.

Safety. The ODB object persistence and query APIs are statically typed. You use C++ iden-

tifiers instead of strings to refer to object members and the generated code makes sure

database and C++ types are compatible. All this helps catch programming errors at

compile-time rather than at runtime.

Revision 1.8, January 20128 C++ Object Persistence with ODB

1.2 Benefits

With the fourth change we make the odb::access class a friend of our person class. This is

necessary to make the default constructor and the data members accessible to the ODB support

code. If your class has public default constructor and public data members, then the friend

declaration is unnecessary.

The final change adds a data member called id_ which is preceded by another pragma. In ODB

every persistent object must have a unique, within its class, identifier. Or, in other words, no two

persistent instances of the same type have equal identifiers. For our class we use an integer id.

The db id auto pragma that precedes the id_ member tells the ODB compiler that the

following member is the object’s identifier. The auto specifier indicates that it is a

database-assigned id. A unique id will be automatically generated by the database and assigned to

the object when it is made persistent.

In this example we chose to add an identifier because none of the existing members could serve

the same purpose. However, if a class already has a member with suitable properties, then it is

natural to use that member as an identifier. For example, if our person class contained some

form of personal identification (SSN in the United States or ID/passport number in other coun-

tries), then we could use that as an id. Or, if we stored an email associated with each person, then

we could have used that since each person is presumed to have a unique email address, for

example:

class person

{

...

#pragma db id

std::string email_;

std::string first_;

std::string last_;

unsigned short age_;

};

Now that we have the header file with the persistent class, let’s see how we can generate that

database support code.

2.2 Generating Database Support Code

The persistent class definition that we created in the previous section was particularly light on any

code that could actually do the job and store the person’s data to a database. There was no serial-

ization or deserialization code, not even data member registration, that you would normally have

to write by hand in other ORM libraries for C++. This is because in ODB code that translates

between the database and C++ representations of an object is automatically generated by the

ODB compiler.

Revision 1.8, January 201212 C++ Object Persistence with ODB

2.2 Generating Database Support Code

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