Why didn’t they add the operator version of iota?

Question

The iota template function has been added to the standard library to populate a range of iterators with an increasing sequence of values.

template<typename ForwardIterator, typename Tp> void iota(ForwardIterator first, ForwardIterator last, Tp value) { for (; first != last; ++first) { *first = value; ++value; } } 

Most other templates in <numeric> have versions that accept user-defined operators. Having this:

  template<typename ForwardIterator, typename Tp, typename Operator> void iota(ForwardIterator first, ForwardIterator last, Tp value, Operator op) { for (; first != last; ++first) { *first = value; op(value); } } 

it would be convenient if you do not want (or cannot) overload the ++ () operator for Tp. I would find this version more widely used than the default ++ () version. <

Answer 1

With lambdas, the second version does not save much, you can just use std::generate .

 template<typename ForwardIterator, typename Tp, typename Operator> void iota(ForwardIterator first, ForwardIterator last, Tp value, Operator op) { std::generate(first, last, [&value,&op](){auto v = value; op(value); return v;}); } 

In fact, this makes the existing std::iota very redundant:

 template<typename ForwardIterator, typename Tp> void iota(ForwardIterator first, ForwardIterator last, Tp value) { std::generate(first, last, [&value](){return value++;}); } 

Answer 2

I suspect the cause is the usual combination of one or more of the following reasons:

• No one sent an offer
• It was not considered important enough for this version (which was already huge and very late)
• it fell through cracks and was forgotten (e.g. copy_if in C ++ 98)
• it is easy to replace using std::generate .