将__string发送到__unsafe_unretained会更改指针的保留/释放属性

Question

以下是我在基于iOS 4（无ARC）的lynda iOS培训课程中学到的代码段。

我打算在我的Xcode 4.2中实现相同的代码，其中使用iOS 5 SDK启用了ARC。它给了我这个错误：

  

错误：语义问题：将'__strong id *'发送到'__unsafe_unretained id **'类型的参数会更改指针的保留/释放属性“

- (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];
    }
}

**真实案例附带了整个函数的以下部分。**

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

我该如何解决这个问题？

Answer 1

这个功能有很多问题。编译器错误的修复只是使用keys所有权限定符声明values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。

由于你根本没有使用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:以取NSArray *而不是va_list。如果您需要帮助，请向我们展示bindSQL:arguments:的源代码。

（但是，我不认为allKeys和allValues会返回并行数组...

Answer 2

编辑const char指针：

- (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];
    }
}

在更新方法中，您应该了解va_list只是一组包含任意数量的参数。假设被调用的方法知道如何确定类型。 NSLog就是一个例子。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传递，即以逗号分隔的值。因为您只有一个值，所以只需传入值即可。我粗略地说sqlite是否想要一个c字符串，或者在这种情况下是否需要一个整数。