adj.cc

#include <bits/stdc++.h>
using namespace std;

typedef complex<int> P;
struct adjacent{
    P p;
    adjacent(const P &p) : p(p){}
    template<typename ...Args> adjacent(const Args &...args) : p(args...){}

    struct Iter : public forward_iterator_tag {
        const P &p;
        int i;
        Iter(const P &p, const int &i) : p(p), i(i){}
        Iter &operator++(){ ++i; return *this; }
        friend bool operator!=(const Iter &a, const Iter &b){
            return a.i != b.i; }
        P operator*(){ return p + P(i&1 ? 0 : i-1, i&1 ? i-2 : 0); }
    };
    Iter begin(){ return Iter(p, 0); }
    Iter end(){ return Iter(p, 4); }
};


bool solve(){
    for(auto p : adjacent(2, 2)) cout << p << endl;
    return true;
}
int main(){
    solve();
    return 0;
}

adj_skip.cc

#include <bits/stdc++.h>
using namespace std;

typedef complex<int> P;
struct adjacent{
    P p;
    adjacent(const P &p) : p(p){}
    template<typename ...Args> adjacent(const Args &...args) : p(args...){}

    struct Iter : public forward_iterator_tag {
        const P &p;
        int i;
        Iter(const P &p, const int &i) : p(p), i(i){skip();}
        void skip(){
            while(i != 4 and (real(**this) < 0 or imag(**this) < 0)) ++i;
        }
        Iter &operator++(){
            ++i;
            skip();
            return *this;
        }
        friend bool operator!=(const Iter &a, const Iter &b){
            return a.i != b.i; }
        P operator*(){ return p + P(i&1 ? 0 : i-1, i&1 ? i-2 : 0); }
    };
    Iter begin(){ return Iter(p, 0); }
    Iter end(){ return Iter(p, 4); }
};


bool solve(){
    for(auto p : adjacent(0, -1)){
        cout << p << endl;
        cout.flush();
    }
    return true;
}
int main(){
    solve();
    return 0;
}

dxdy.cc

#include <bits/stdc++.h>
#define aur auto&
using namespace std;

struct{
    struct Iter : public forward_iterator_tag {
        int i;
        Iter(const int &i) : i(i){}
        Iter &operator++(){ ++i; return *this; }
        friend bool operator!=(const Iter &a, const Iter &b){
            return a.i != b.i;
        }
        pair<int, int> operator*(){
            return make_pair(i&1 ? 0 : i-1, i&1 ? i-2 : 0);
        }
    };
    Iter begin(){ return Iter(0); }
    Iter end(){ return Iter(4); }
} dirs;

signed main(){
    for(auto dir : dirs){
        int dx, dy; tie(dx, dy) = dir;
        cout << dx << ", " << dy << endl;
    }
    return 0;
}
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