今天看freshmeat的時(shí)候，又發(fā)現(xiàn)一個(gè)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(gè)Persistent類(lèi)的步驟如下：
一、繼承 Persistent
class OwnPersistent : public Persistent {};

二、使用宏P(guān)ERSISTENT_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(gè)參數(shù)是類(lèi)名，第二個(gè)參數(shù)表示后面有多少個(gè)變量，后面兩個(gè)兩個(gè)參數(shù)為一組，描述每一個(gè)變量。

三、可以用RELATIONS宏描述類(lèi)之間關(guān)聯(lián)
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(shuō)從哪個(gè)類(lèi)映射到哪個(gè)類(lèi)
relnid是映射關(guān)系的編號(hào)
relnbidir是指定映射是否雙向（只有當(dāng)from和to是同一個(gè)類(lèi)的時(shí)候可以使用）
relnname映射關(guān)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(shù)了，這里看得不是很明白，呵呵。

最后的結(jié)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(guò)沒(méi)有仔細(xì)看具體實(shí)現(xiàn)。按作者自己說(shuō)難度之一就是實(shí)現(xiàn)這種不定參數(shù)的宏。