将__string发送到__unsafe_unretained更改保留/释放指针的属性

Question

下面是我在林达iOS基于iOS 4的培训课程中学到的一个代码片段。

我打算在我的Xcode 4.2中实现相同的代码，Xcode 4.2是用iOS 5 SDK打开的。它给了我一个错误：

错误:语义问题:将“__strong id *”发送到'__unsafe_unretained id **‘类型的参数，更改保留/释放指针的属性“

- (NSNumber *) insertRow:(NSDictionary *) record {
    // NSLog(@"%s", __FUNCTION__);
    int dictSize = [record count];

    // the values array is used as the argument list for bindSQL
    id keys[dictSize];  // not used, just a side-effect of getObjects:andKeys
    id values[dictSize];
    [record getObjects:values andKeys:keys];    // convenient for the C array

    // construct the query
    NSMutableArray * placeHoldersArray = [NSMutableArray arrayWithCapacity:dictSize];
    for (int i = 0; i < dictSize; i++)  // array of ? markers for placeholders in query
        [placeHoldersArray addObject: [NSString stringWithString:@"?"]];

    NSString * query = [NSString stringWithFormat:@"insert into %@ (%@) values (%@)",
                        tableName,
                        [[record allKeys] componentsJoinedByString:@","],
                        [placeHoldersArray componentsJoinedByString:@","]];

    [self bindSQL:[query UTF8String] arguments:(va_list)values];
    sqlite3_step(statement);
    if(sqlite3_finalize(statement) == SQLITE_OK) {
        return [self lastInsertId];
    } else {
        NSLog(@"doQuery: sqlite3_finalize failed (%s)", sqlite3_errmsg(database));
        return [NSNumber numberWithInt:0];
    }
}

**实际情况是整个功能的以下部分。

int dictSize = [record count];

// the values array is used as the argument list for bindSQL
id keys[dictSize];  // not used, just a side-effect of getObjects:andKeys
id values[dictSize];
[record getObjects:values andKeys:keys];    // convenient for the C array

我怎么解决这个问题？

Answer 1

这个功能有很多问题。编译器错误的修正是使用keys所有权限定符声明__unsafe_unretained和__unsafe_unretained：

__unsafe_unretained id keys[dictSize];  // not used, just a side-effect of getObjects:andKeys
__unsafe_unretained id values[dictSize];

但是，强制转换(va_list)values是未定义的行为。不要这样做。

由于您根本没有使用keys数组，所以最好这样做：

- (NSNumber *) insertRow:(NSDictionary *) record {
    int count = [record count];

    NSMutableArray * placeHoldersArray = [NSMutableArray arrayWithCapacity:count];
    for (int i = 0; i < count; i++) {
        [placeHoldersArray addObject: @"?"];
    }

    NSString * query = [NSString stringWithFormat:@"insert into %@ (%@) values (%@)",
                        tableName,
                        [[record allKeys] componentsJoinedByString:@","],
                        [placeHoldersArray componentsJoinedByString:@","]];

    [self bindSQL:[query UTF8String] arguments:[record allValues]];
    sqlite3_step(statement);
    if(sqlite3_finalize(statement) == SQLITE_OK) {
        return [self lastInsertId];
    } else {
        NSLog(@"doQuery: sqlite3_finalize failed (%s)", sqlite3_errmsg(database));
        return [NSNumber numberWithInt:0];
    }
}

然后修改bindSQL:arguments:以接受NSArray *而不是va_list。如果需要帮助，请向我们展示bindSQL:arguments:的源代码。

(但是，我不认为allKeys和allValues返回并行数组(…)是有记载的。)

Answer 2

为const char指针编辑：

- (NSNumber *) insertRow:(NSDictionary *) record {
    NSArray * placeHoldersArray = [NSArray array];
    const char * cStrings[record.count];
    NSArray * keys = [record allKeys];
    NSUInteger i=0;
    for (id key in keys) {
        id object = [record objectForKey:key];
        if ([object isKindOfClass:[NSString class]])
            cStrings[i++] = [(NSString*)object UTF8String];
        else
            @throw [NSException exceptionWithName:NSInvalidArgumentException reason:@"Expected NSString" userInfo:nil];
        placeHoldersArray = [placeHoldersArray arrayByAddingObject: @"?"]; 
    }    
    NSString * query = [NSString stringWithFormat:@"insert into %@ (%@) values (%@)",
                            tableName,
                            [keys componentsJoinedByString:@","],
                            [placeHoldersArray componentsJoinedByString:@","]];

    [self bindSQL:[query UTF8String] arguments:(va_list)cStrings];
    sqlite3_step(statement);
    if(sqlite3_finalize(statement) == SQLITE_OK) {
        return [self lastInsertId];
    } else {
        NSLog(@"doQuery: sqlite3_finalize failed (%s)", sqlite3_errmsg(database));
        return [NSNumber numberWithInt:0];
    }
}

在update方法中，您应该理解va_list只是一组任意数量的参数。假定被调用的方法知道如何确定类型。这方面的一个例子是NSLog。它使用格式字符串来确定未知数量的变量的类型，这些变量作为va_list传入，即逗号分隔的值。因为你只有一个值，就把这个值传递给你。我还不清楚sqlite是否需要c字符串，或者在这种情况下它是否需要一个整数。