Declaring run time variables

[splatterdash via Digitalmars-d-learn]

On Monday, 4 August 2014 at 22:09:49 UTC, anonymous wrote:
> On Monday, 4 August 2014 at 22:00:18 UTC, splatterdash wrote:
>> ```
>> File f = File("input_file")
>> // detect gzip ...
>> if (isGzip)
> {
>>    auto fileIter = new MyFileReader!GzipIterator(f)
> }
>> else
> {
>>    auto fileIter = new MyFileReader!NormalIterator(f)
> }
>>
>> foreach(string line; fileIter) {
>> // do things
>> }
>> ```
>
> (Added braces for clarity.)
>
>> However I always get the compiler error `undefined identifier 
>> lineIter`.
>
> You have two independent variables called fileIter. Both are 
> only
> available in their respective scopes. Declare the variable 
> before
> the `if` to get one variable with a wider scope:
>
> SomeCommonBaseTypeOfThoseMyFileReaderVariants fileIter;
> if (isGzip)
>      fileIter = new MyFileReader!GzipIterator(f);
> else
>      fileIter = new MyFileReader!NormalIterator(f);

Indeed I do. I'm not sure which type I should use for the common 
base type, though. MyFileReader is a templated class, so using it 
plainly did not work. I also tried `InputRange!string` to no 
avail despite `MyFileReader` implementing the three InputRange 
requirement (popFront(), front, and empty).

Any ideas on what I should as the class?