今天看freshmeat的时候，又发现一个有趣Object Persistent方案。
http://litesql.sourceforge.net/

Features

• C++ wrapper for sqlite
• all the good stuff of sqlite
• light persistence layer with relation support
• automatic database schema creation and upgrading from C++ classes
• small code base: less than 2500 lines of C++
• create complex SQL queries using compile-time checked class API; minimizes need to write SQL query strings

按文档，做一个Persistent类的步骤如下：
一、继承 Persistent
class OwnPersistent : public Persistent {};

二、使用宏PERSISTENT_BASE声明哪些变量需要Persistent
class OwnPersistent : public Persistent {
    PERSISTENT_BASE(OwnPersistent, 1, name, TEXT);
};
其中，第一个参数是类名，第二个参数表示后面有多少个变量，后面两个两个参数为一组，描述每一个变量。

三、可以用RELATIONS宏描述类之间关联
RELATIONS(object, relnum,
         rel1type, rel1from, rel1to, rel1id, rel1bidir, rel1name,
         rel2type, rel2from, rel2to, rel2id, rel2bidir, rel2name, ...)

其中的 relntype可以为以下几种：
OORelation : one-to-one relation
OMRelation : one-to-many relation
MORelation : many-to-one relation
MMRelation : many-to-many relation

relnfrom、relnto是说从哪个类映射到哪个类
relnid是映射关系的编号
relnbidir是指定映射是否双向（只有当from和to是同一个类的时候可以使用）
relnname映射关系的名字

例如：
class Person : public Persistent {
    PERSISTENT_BASE(Person, 1, name, TEXT);
    RELATIONS(Persistent, 3,
              OORelation, Person, Person, 1, false, mother,
              OORelation, Person, Person, 2, false, father,
              MMRelation, Person, Person, 3, true, friends);
};

最后就是声明cleanUp函数了，这里看得不是很明白，呵呵。

最后的结果就是：
class Person : public Persistent {
    PERSISTENT_BASE(Person, 1, name, TEXT);
    RELATIONS(Persistent, 3,
              OORelation, Person, Person, 1, false, mother,
              OORelation, Person, Person, 2, false, father,
              MMRelation, Person, Person, 3, true, friends);
     virtual void cleanUp() {
         mother.flush();
         father.flush();
         friends.flush();
     }
};

Person bill(db), bob(db);
bill.name = "Bill";
bill.update();

bob.name = "Bob";
bob.update();
// both objects must be stored in database before they can be linked
bill.friends.link(bob);
// following statement would throw an exception because they are already friends
bob.friends.link(bill);

Person bob = bill.friends.fetchOne(Person::name_() == "Bob");
vector billsFriends = bill.friends.fetch();

效果的确有趣。不过没有仔细看具体实现。按作者自己说难度之一就是实现这种不定参数的宏。