只有超类才能访问ivar _species 。您的子类应如下所示：

Only the superclass has access to the ivar _species. Your subclass should look like this:

- (NSString *)species {
    NSString *value = [super species];
    if (!value) {
        self.species = @"Homo sapiens";
    }

    return [super species];
}

如果当前未设置值，则将值设置为默认值。另一个选择是：

That sets the value to a default if it isn't currently set at all. Another option would be:

- (NSString *)species {
    NSString *result = [super species];
    if (!result) {
        result = @"Home sapiens";
    }

    return result;
}

如果没有值，则不会更新该值。它只是根据需要返回默认值。

This doesn't update the value if there is no value. It simply returns a default as needed.