Why can't I do std::map.begin() + 1?

Question

I have a `std::map`, which I want to iterate over, starting at the second entry. I can workaround this fine, but I'm confused about why the "obvious" syntax doesn't compile. The error message doesn't help, because it refers to `std::string`, which I'm not using here. Here's some code ``` // Suppose I have some map ... std::map pSomeMap; // This is fine ... std::map::const_iterator pIterOne = pSomeMap.begin(); ++pIterOne; // This doesn't compile ... std::map::const_iterator pIterTwo = pSomeMap.begin() + 1; ``` Visual Studio 2012 gives the following error on the above line: ``` error C2784: 'std::_String_iterator<_Mystr> std::operator + (_String_iterator<_Mystr>::difference_type,std::_String_iterator<_Mystr>)' : could not deduce template argument for 'std::_String_iterator<_Mystr>' from 'int' ``` What's happening here?

Accepted Answer

`std::map::iterator` is of the iterator-class bidirectional iterator. Those only have `++` and `--` operators. `+N` and `[]` is only available for random access iterators (which can be found in e.g. `std::vector`). The reason behind this is that adding `N` to a random access iterator is constant time (e.g. add `N*sizeof(T)` to a `T*`), whereas doing the same thing for a bidirectional iterator would require applying `++` `N` times. What you can do though (if you have C++11) is: ``` std::map::const_iterator pIterTwo = std::next(pSomeMap.begin(),1); ``` which does the right thing for all iterator types.

Why can't I do std::map.begin() + 1?

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