template<class T>
struct output_span_impl{
    void* ud;
    void (*init)(void*, T** beg, T** mid, T** end);
    void (*write_back)(void*, T* mid);
    void (*grow)(void*, T** beg, T** mid, T** end);
};


template<class T>
struct output_span{
    output_span(output_span_impl<T> impl):
        impl(impl){
        impl(impl.ud, &beg, &mid, &end);
    }

    ~output_span(){
        impl.write_back(impl, mid);
    }

    void push_back(T val){
        if (mid == end){
            impl.grow(impl.ud, &beg, &mid, &end);
        }
        *mid = val;
        mid++;
    }

    ...

    output_span_impl<T> impl;
    T* beg;
    T* mid;
    T* end;
};