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From time to time I am feeling the need for a certain kind of iterator (for which I can't make up a good name except the one prefixed to the title of this question). Suppose we have a function (or function object) that maps an integer to type T. That is, we have a definition of a mathematical sequence, but we don't actually have it stored in memory. I want to make an iterator out of it. The iterator class would look something like this: template <class F, class T> class sequence_iterator : public std::iterator<...> { int i; F f; public: sequence_iterator (F f, int i = 0):f(f), i(i){} //operators ==, ++, +, -, etc. will compare, increment, etc. the value of i. T operator*() const { return f(i); } }; template <class T, class F> sequence_iterator<F, T> make_sequence_iterator(F f, int i) { return sequence_iterator<F, T>(f, i); } Maybe I am being naive, but I personally feel that this iterator would be very useful. For example, suppose I have a function that checks whether a number is prime or not. And I want to count the number of primes in the interval [a,b]. I'd do this; int identity(int i) { return i; } count_if(make_sequence_iterator<int>(identity, a), make_sequence_iterator<int>(identity, b), isPrime); Since I have discovered something that would be useful (at least IMHO) I am definitely positive that it exists in boost or the standard library. I just can't find it. So, is there anything like this in boost? . In the very unlikely event that there actually isn't, then I am going to write one - and in this case I'd like to know your opinion whether or not should I make the iterator_category random_access_iterator_tag . My concern is that this isn't a real RAI, because operator* doesn't return a reference.</p

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