动态分配指针和指针变量的数据类型

Question

任何人都可以清楚地解释（带有示例）为什么我们必须将动态分配指针的数据类型从默认的void类型更改为其相应指针变量的相同数据类型？

例如：

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    int groupingSize = 2;
    if([string length] == 0) {
        groupingSize = 4;
    }
    NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init] ;
    NSString *separator = @"-";
    [formatter setGroupingSeparator:separator];
    [formatter setGroupingSize:groupingSize];
    [formatter setUsesGroupingSeparator:YES];
    [formatter setSecondaryGroupingSize:2];
    if (![string  isEqual: @""] && (textField.text != nil && textField.text.length > 0)) {
        NSString *num = textField.text;
        num = [num stringByReplacingOccurrencesOfString:separator withString:@""];
        NSString *str = [formatter stringFromNumber:[NSNumber numberWithDouble:[num doubleValue]]];
        textField.text = str;
    }
    return YES;
}