LRU Cache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

class LRUCache{
private:
    struct item_t{
        int key, val;
        item_t(int k, int v) :key(k), val(v){}
    };
    typedef list<item_t> list_t;
    typedef unordered_map<int, list_t::iterator> map_t;

    map_t   m_map;
    list_t  m_list;
    int     m_capacity;
public:
    LRUCache(int capacity) : m_capacity(capacity) {
    }
    int get(int key) {
        map_t::iterator i = m_map.find(key);
        if (i == m_map.end()) return -1;
        m_map[key] = promote(i->second);
        return m_map[key]->val;
    }
    void set(int key, int value) {
        map_t::iterator i = m_map.find(key);
        if (i != m_map.end()){
            m_map[key] = promote(i->second);
            m_map[key]->val = value;
        }
        else {
            if (m_map.size() < m_capacity){
                m_map[key] = m_list.insert(m_list.end(), item_t(key, value));
            }
            else {
                m_map.erase(m_list.front().key);
                m_list.pop_front();
                m_map[key] = m_list.insert(m_list.end(), item_t(key, value));
            }
        }
    }
    list_t::iterator promote(list_t::iterator i){
        list_t::iterator inew = m_list.insert(m_list.end(), *i);
        m_list.erase(i);
        return inew;
    }
};