How to check that the passed Iterator is a random access iterator?

Question

I have the following code, which does some iterator arithmetic:

template<class Iterator> void Foo(Iterator first, Iterator last) {   typedef typename Iterator::value_type Value;   std::vector<Value> vec;   vec.resize(last - first);   // ... } 

The (last - first) expression works (AFAIK) only for random access iterators (like the ones from vector and deque). How can I check in the code that the passed iterator meets this requirement?

Answer 1

If Iterator is a random access iterator, then

std::iterator_traits<Iterator>::iterator_category 

will be std::random_access_iterator_tag. The cleanest way to implement this is probably to create a second function template and have Foo call it:

template <typename Iterator> void FooImpl(Iterator first, Iterator last, std::random_access_iterator_tag) {      // ... }  template <typename Iterator> void Foo(Iterator first, Iterator last) {     typedef typename std::iterator_traits<Iterator>::iterator_category category;     return FooImpl(first, last, category()); } 

This has the advantage that you can overload FooImpl for different categories of iterators if you'd like.

Scott Meyers discusses this technique in one of the Effective C++ books (I don't remember which one).