Linear search algorithm template implementation
Returning int is dubious; it unnecessarily narrows the range of possible return values. Consider retuning an iterator.
The line if (_list[i] == _item) implies that _list must provide an operator[](int), which is very restrictive. A linear search is just like its name implies, linear. It is expected to work on any linear collection, e.g. forward iterator (maybe even on input iterator).
The class sLinear exposes one public method, and keeps no state. There's no reason to make it a class. A standalone

template &lt;typename I&gt;
I findItem(I first, I last, I::value_type value) {
    ....
}


will do as well.
while (_list != nullptr) is very strange. _list never changes; why do you want to loop?

PS: Could you explain a rationale for returning i+1?