今天看freshmeat的時候，又發現一個有趣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

按文檔，做一個Persistent類的步驟如下：
一、繼承 Persistent
class OwnPersistent : public Persistent {};

二、使用宏PERSISTENT_BASE聲明哪些變量需要Persistent
class OwnPersistent : public Persistent {
??? PERSISTENT_BASE(OwnPersistent, 1, name, TEXT);
};
其中，第一個參數是類名，第二個參數表示后面有多少個變量，后面兩個兩個參數為一組，描述每一個變量。

三、可以用RELATIONS宏描述類之間關聯
RELATIONS(object, relnum,
???????? rel1type, rel1from, rel1to, rel1id, rel1bidir, rel1name,
?????????rel2type, rel2from, rel2to, rel2id, rel2bidir, rel2name, ...)

其中的 relntype可以為以下幾種：
OORelation : one-to-one relation
OMRelation : one-to-many relation
MORelation : many-to-one relation
MMRelation : many-to-many relation

relnfrom、relnto是說從哪個類映射到哪個類
relnid是映射關系的編號
relnbidir是指定映射是否雙向（只有當from和to是同一個類的時候可以使用）
relnname映射關系的名字

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);
};

最后就是聲明cleanUp函數了，這里看得不是很明白，呵呵。

最后的結果就是：
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();

效果的確有趣。不過沒有仔細看具體實現。按作者自己說難度之一就是實現這種不定參數的宏。