C++ CRTP neuron example

#include <vector>
#include <iostream>
using namespace std;

struct Neuron;

template <typename Self>
struct SomeNeurons {
    // must be declaration here only
    // implementation should be placed at the end
    // otherwise there will be 'incomplete type' issue
    template <typename T> void connect_to(T& other);
};

struct Neuron : SomeNeurons<Neuron> {
    vector<Neuron*> in, out;
    unsigned int id;

    Neuron()
    {
        static int id = 1;
        this->id = id++;
    }

    Neuron* begin()
    {
        return this;
    }
    Neuron* end()
    {
        return this + 1;
    }

    friend ostream& operator<<(ostream& os, const Neuron& obj)
    {
        for (Neuron* n : obj.in) {
            os << n->id << "\t-->\t[" << obj.id << "]" << endl;
        }

        for (Neuron* n : obj.out) {
            os << "[" << obj.id << "]\t-->\t" << n->id << endl;
        }
        return os;
    }
};

struct NeuronLayer : vector<Neuron>, SomeNeurons<NeuronLayer> {
    NeuronLayer(int count)
    {
        while (count-- > 0)
            emplace_back(Neuron{});
    }

    friend ostream& operator<<(ostream& os, NeuronLayer& obj)
    {

        for (auto& n : obj) os << n;
        return os;
    }
};

// place template implementation here at the end
template <typename Self>
template <typename T>
void SomeNeurons<Self>::connect_to(T& other)
{
    for (Neuron& from : *static_cast<Self*>(this)) {
        for (Neuron& to : other) {
            from.out.push_back(&to);
            to.in.push_back(&from);
        }
    }
}

int main()
{
    Neuron n1;
    NeuronLayer l2{2}, l3{3};
    n1.connect_to(l2);
    l2.connect_to(l3);
    cout << "Neuron n1" << endl << n1;
    cout << "Layer l2" << endl << l2;
    cout << "Layer l3" << endl << l3;

    return 0;
}