从未定义结构 [英] Never defined structure

问题描述

有没有在具有C从不定义的结构有没有优惠？

Is there any benefit in having never-defined structures in C ?

例在SQLite的源$ C ​​$ C：

Example in SQLite source code :

/* struct sqlite3_stmt is never defined */
typedef struct sqlite3_stmt sqlite3_stmt;

和对象被操纵，像这样：

And the object is manipulated like so :

typedef struct Vdbe Vdbe;
struct Vdbe {
    /* lots of members */
};


int sqlite3_step(sqlite3_stmt *pStmt) {
    Vdbe *v = (Vdbe*) pStmt;
    /* do stuff with v... */
}

那么，为什么不直接使用通常的抽象类型，在 foo.c的私下定义的实际来源结构和公共的typedef 在 foo.h中头？

So why not just use a usual abstract type, with the actual structure privately defined in foo.c source and a public typedef in foo.h header ?

推荐答案

要澄清：什么你问的就是为什么SQLite的确实不是这样做上面的：

To clarify: What you're asking is why SQLite does the above instead of doing this:

头文件：

typedef struct sqlite3_stmt sqlite3_stmt;

C文件：

struct sqlite3_stmt {
    /* lots of members */
};


int sqlite3_step(sqlite3_stmt *pStmt) {
    /* do stuff with pStmt... */
}

（这是KennyTM的回答联系到不透明指针模式的规范形式。）

(This is the canonical form of the "opaque pointer" pattern linked to in KennyTM's answer.)

唯一的好理由，我能想到的，为什么SQLite的做什么它做的是以下内容：

The only good reason I can think of why SQLite does what it does is the following:

后端code，我猜测，这个API来之前和使用的名称 VDBE - 这个名字可能意味着相关沿实施的东西虚拟数据库条目的线路（疯狂猜测这里）。

The backend code, I'm speculating, came before the API and used the name Vdbe -- the name probably means something related to the implementation along the lines of "virtual database entry" (guessing wildly here).

当时间到了创建API，有人意识到，需要通过 sqlite3_step 参数是一个 VDBE 但这是不完全，将传达很多API的用户的名称。因此，从用户的角度来看，一个 VDBE 被称为 sqlite3_stmt 。

When time came to create the API, someone realized that the parameter required by sqlite3_step was a Vdbe but that this was not exactly a name that would convey a lot to the user of the API. Hence, from the user's point of view, a Vdbe is referred to as an sqlite3_stmt.

这里的关键，那么，两个视图在同一项目的区分：后端认为在 VDBE S（无论他们是）方面，因为这是一个名字这使得在实施的背景下意义。关于 sqlite3_stmt 是因为这是有意义的背景下一个名称的接口的。

The point here, then, is to differentiate between two views of the same item: The backend thinks in terms of Vdbes (whatever they are) because that's a name that makes sense in the context of the implementation. The API talks about sqlite3_stmts because that's a name that makes sense in the context of the interface.

编辑：由于Amarghosh指出，为什么不这样做，以达到同样的效果。

As Amarghosh points out, why not just do this to achieve the same effect?

typedef struct Vdbe sqlite3_stmt;

KennyTM指出，一个好的可能的原因（请投他，我不不想在这里吸走他的REP）：VDBE只有几种可能的后端之一;接口采用了通用 sqlite3_stmt ，这是再投对什么后端使用来实现它。

KennyTM points out a good possible reason (please vote him up, I don't want to siphon off his rep here): VDBE is only one of several possible backends; the interface uses a "generic" sqlite3_stmt, and this is then cast to whatever the backend uses to implement it.

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