Converting a string to an integer (could use more work)

gistfile1.cpp

#include <iostream>
#include <locale>

template <class Facet>
struct erasable_facet : Facet
{
    erasable_facet() : Facet(0) { }
    ~erasable_facet() { }
};

void insert_num(const std::string& s, int& n)
{
    erasable_facet<std::num_get<char, std::string::const_iterator>> facet;
    std::ios str(nullptr);
    std::ios_base::iostate err = std::ios_base::goodbit;
    long l;
    
    facet.get(s.begin(), s.end(), str, err, l);
    
    if (!(err & std::ios_base::failbit) && !(err & std::ios_base::badbit))
        n = l;
}

int main()
{
    std::string str = "123";
    int n;
    
    insert_num(str, n);
    std::cout << n;
}