Question

I have a custom read-only data structure that I need to transverse. I would like to create a custom iterator that need to skip certain values.

A simple but equivalent example would be the following. I have a vector of numbers and I want to loop through all skipping the negative values. Normally I would do something like:

vector<int> v;
for (vector<int>::iterator it = v.begin(); it!=v.end(); ++it) {
    if (*it > 0) {
       dosomething(*it);
    }
}

But I would like to do something like:

vector<int> v;
for (vector<int>::my_iterator it = v.my_begin(); it!=v.my_end(); ++it) {
    dosomething(*it);
}

What is the right way to achieve this?

Answer 1

This is easily achieved with a boost::filter_iterator, if your data structure already stores a container under the hood. Here’s a simple example:

#include <vector>
#include <iostream>
#include <boost/iterator/filter_iterator.hpp>

class X{
    typedef std::vector<int> container;
    struct Pred{
        bool operator()(int i){
            return i % 2 == 0;
        }
    };

public:
    typedef boost::filter_iterator<Pred, container::iterator> iterator;

    void add(int val){ nums.push_back(val); }
    iterator begin(){ return iterator(nums.begin(), nums.end()); }
    iterator end(){ return iterator(nums.end(), nums.end()); }

private:
    container nums;
};

int main(){
    X x;
    for(int i=0; i < 10; ++i)
        x.add(i);
    for(X::iterator it = x.begin(), ite = x.end(); it != ite; ++it)
        std::cout << *it << ' ';
}

Live example at Ideone. Output:

0 2 4 6 8