1.6.5 value_type in container

• value_type in container.
  vector<uint> vecs; 
  cout << sizeof(vecs.value_type)  //error usage 
  cout<<sizeof(vector<uint>::value_type); 
  //Pay attention to ::
• Having the commonly-used types available as a type on the container is useful when the container’s type itself is unknown. For example, someone may want to write library code that works equally well with std::map and std::unordered_map:
  template<typename TMap> 
  void insert_default_pair(TMap& map) 
  { 
      map.emplace(typename TMap::key_type(), 
                         typename TMap::mapped_type()); 
  } 
  share
• inside templated code, prefix value_type with the keyword typename. Why, It depending on whether some identifier designates a type or a variable, e.g.T * p may be a multiplication or a pointer declaration. Not explicitly marked as type by prefixing it with typename is considered a variable.
  template <typename T> 
  class TSContainer { 
  private: 
          T container; 
  public: 
          void push(_______________typename T::value_type& item){ 
                  container.push_back(item); 
          }
• summary:
  1. complex type
  2. change a lot, in above example
  3. Inside of a function
  4. previous two example, unknow container in template class or support two container at the same time, to reach generic purpose.
  //1)it will save a lot of typing. 
   typedef vector< pair<int, string> > ComVec; 
  ComVec::value_type aaa; 
   
  //2)int to float, but below code don’t need change at all. 
  typedef vector< pair<float, string> > ComVec; 
   
  //3) inside of funciton. 
  typedef std::vector< std::pair<int,std::string> > Record_t; 
  //______________________________________________________________________________ typedef is your good friend, reuse it below 
  Record_t k1; 
   
  int find_it(std::string value, __________________Record_t const& stuff){ 
    auto fit = std::find_if(stuff.begin(), stuff.end(), 
              [value](__________________________________________Record_t::value_type const& vt) -> bool 
                   { return vt.second == value; });