C ++中的通用模板

Question

I don't understand the answers to the following questions:

Write a C++ function find_elem that takes two iterators first and last of some sequence of elements of type T and an object obj of type T. It returns the iterator to the first occurrence of obj in the range [first, last), or the iterator last if obj is not in the sequence.

This was the answer

template <typename It, typename T>
It find_elem(It first, It last, const T & obj) {
   while (first != last && (*first) != obj) // I DON'T UNDERSTAND THIS LINE
   ++first;
   return first;
}

I dont understand the following line while (first != last && (*first) != obj) . Why is it (*first != obj) when the questions asks you to return the iterator with the first instance of obj . I also don't get the following line ++first as in why you are incrementing the iterator first

Answer 1

The ++first is executed by the while loop. I would write use { } here to make it clearer:

while (first!=last && (*first)!=obj) {
   ++first;
}

So, the while loop checks if (*first)==obj. If not, then it moves to the next element in the list using ++first, which increments the iterator. Then it ends either when first==last (meaning that we have gone through the entire list), or when (*first)==obj, meaning that we found what we were looking for.

Answer 2

A copy of first is passed to the function. This means that the function can safely modify the variable by using it also to iterate the sequence.

It's just a concise alternative for the following code:

template <typename It, typename T>
It find_elem(It first, It last, const T & obj) {
   It iterator = first;
   while (iterator != last && (*iterator) != obj)
   ++iterator;
   return iterator;
}

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By the way... "generic templates" sounds strange, because templates are always generic. I suppose template template parameters could be called "generic templates", though.