Question

我只是想以一种简单的if方式决定我要使用哪个文本。为什么X代码会抱怨而不是让我构建项目，说明变量在if中明确定义时是未定义的？

 if(indexPath.row == [listOfItems count]) {
            NSString *cellValue = [[NSString alloc] initWithFormat:@"Totalt: %@st registrerade", companyTotalReg]; //cellValue is unused
        } else {
            NSString *cellValue = [[NSString alloc] initWithFormat:@"Totalt: SEK%@ intjänat", companyTotalPay]; //cellValue is unused
        }
        cell.textLabel.text = cellValue; //cellValue is undefined

Answer 1

你需要这样做：

NSString *cellValue = nil;
if(indexPath.row == [listOfItems count]) {
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: %@st registrerade", companyTotalReg]; //cellValue is unused
} else {
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: SEK%@ intjänat", companyTotalPay]; //cellValue is unused
}
cell.textLabel.text = cellValue; //cellValue is not undefined anymore

否则，从理论上讲，你的if子句都会失败（即使在你的情况下这是不可能的）并且cellValue仍未声明。由于编译器无法知道理论上是否可能使所有条件都失败，所以无论如何它都会警告你。

一般情况下，你应该/必须始终初始化将要使用它们的变量within the scope 。对于cellValue

，cell.textLabel.text = cellValue; 将超出范围。

有点偏离主题，但you should also use NSLocalizedString()代表任何UI字符串，而不是硬编码字符串。

Answer 2

 NSString *cellValue = NULL; 

if(indexPath.row == [listOfItems count]) {
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: %@st registrerade", companyTotalReg]; //cellValue is unused
} else {
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: SEK%@ intjänat", companyTotalPay]; //cellValue is unused
}
cell.textLabel.text = [cellValue autorelease]; // The autorelease is here because you are leaking the memory otherwise. If you release the string later anyway, you can and should remove it!

Answer 3

你这样做的方式定义了两个名为cellValue的NSStrings。第一个cellValue的生命周期仅限于if子句的范围，第二个cellValue的生命周期仅限于else子句的范围。您可以通过两种方式解决此问题：

NSString *cellValue;
if (indexPath.row == [listOfItems count])
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: %@st registrerade", companyTotalReg];
else
    cellValue = [[NSString alloc] initWithFormat:@"Totalt: SEK%@ intjänat", companyTotalPay]; 
cell.textLabel.text = cellValue;

或

NSString *cellValue = (indexPath.row == [listOfItems count])
    ? [[NSString alloc] initWithFormat:@"Totalt: %@st registrerade", companyTotalReg]
    : [[NSString alloc] initWithFormat:@"Totalt: SEK%@ intjänat", companyTotalPay];
cell.textLabel.text = cellValue;

if-case内部创建NSString失败？

3 个答案: