如何调用 sqlite3_open(通过引用传递不透明指针) [英] How to call sqlite3_open (passing opaque pointers by reference)

问题描述

作为练习，我正在尝试为 sqlite3 创建一个包装器.我已经设置了桥接头，并且可以看到 sqlite3 函数的工具提示，但是我不知道如何调用 sqlite3_open

As an exercise, I'm trying to create a wrapper for sqlite3. I've got the bridging header set up, and I can see the tool tips for the sqlite3 functions, but I can't figure out how to call sqlite3_open

sqlite3.h 包含以下 sqlite3 和 sqlite3_open 的定义:

sqlite3.h contains the following definitions of sqlite3 and sqlite3_open:

typedef struct sqlite3 sqlite3;

SQLITE_API int sqlite3_open(
  const char *filename,   /* Database filename (UTF-8) */
  sqlite3 **ppDb          /* OUT: SQLite db handle */
);

这意味着 sqlite3_open 将指向匿名结构的指针作为尾随参数，这在工具提示中似乎很清楚:

Which means that sqlite3_open takes as a trailing parameter a pointer to a pointer to an anonymous structure, which seems clear enough in the tooltip:

func sqlite3_open(filename: CString, ppDb: CMutablePointer<COpaquePointer>) -> CInt

知道CMutablePointer的意思是传入&T，我来的最接近的是:

Knowing that CMutablePointer means to pass in &T, the closest I've come is:

class Database {
    var handle:COpaquePointer

    init(file:String) {
        let error = sqlite3_open(file as CString, &handle)
    }

    deinit {
        sqlite3_close(handle)
    }
}

sqlite3_close 行没有错误，所以我认为我至少接近了，但是 sqlite3_open 行产生了:

There's no error on the sqlite3_close line, so I think I'm at least close, but the sqlite3_open line yields:

Cannot convert the expression's type 'CInt' to type '$T9'

有关如何执行此操作的任何线索?

Any clues on how to do this?

请不要回答说使用 FMDB 或其他基于 Objective-C 的接口.正如我所说，这至少部分是弄清楚如何从 swift 使用 C 库的练习.

Please, no answers that say to use FMDB or other Objective-C based interfaces. As I said, this is at least partially an exercise in figuring out how to use C libraries from swift.

推荐答案

问题不在于句柄参数，而在于字符串转换.以下作品...

The problem is not with the handle parameter, but with the string conversion. The following works…

class Database {
    var handle: COpaquePointer = nil

    init(file: NSString) {
        let error = sqlite3_open(file.cStringUsingEncoding(NSUTF8StringEncoding), &handle)
    }
}

我不确定为什么as CString"不起作用.

I'm unsure as to why the 'as CString' doesn't work.

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