C++ 模板中的型別獲取

1. 型別判斷

• 嚴格型別比較：std::is_same<T1, T2>::value

  cout << std::is_same<int, int>::value << endl; // true
  cout << std::is_same<int, long>::value << endl; // false
  cout << std::is_same<int, unsigned int>::value << endl; // false
  cout << std:
   
  :is_same<int, const int>::value << endl; // false
  cout << std::is_same<int, int&>::value << endl; // false
  cout << std::is_same<int, int*>::value << endl; // false

• 退化型別比較：std::decay<T>::type

  cout << std::is_same<int, std::decay<int>
   
  ::type>::value << endl; // ture
  cout << std::is_same<int, std::decay<long>::type>::value << endl; // false
  cout << std::is_same<int, std::decay<unsigned int>::type>::value << endl; // false
  cout << std::is_same<int, std::decay<const int>:
   
  :type>::value << endl; // true
  cout << std::is_same<int, std::decay<int&>::type>::value << endl; // ture
  cout << std::is_same<int, std::decay<int*>::type>::value << endl; // false

2. 容器元素型別

• STL 的容器支援： Container::value_type

  using type = Container::value_type;

3. 迭代器型別

• 標頭檔案：#include <type_traits>
• using type = std::iterator_traits<Itr>::value

4. 函式返回型別

• std::result_of<Func()>::type

5. 舉兩栗子

• 判斷STL容器是否包含某元素：

  • 當容器元素為基本型別時：

  // map::value_type is const std::pair<const T, T>
  // need reload operator "==" to pass compile
  template <class T>
  bool operator==(const pair<const T,T> p1, const pair<T,T> p2){
      return p1.first==p2.first && p1.second==p2.second;
  }
  
  // basic value type
  template <class Container, class ItemType>
  int isContain(Container const &container, ItemType const &item){
      using type = typename std::decay<typename Container::value_type>::type;
      bool isSame = std::is_same<type, typename std::decay<ItemType>::type>::value;
      if(!isSame){
          cout << "error type not same " << endl;
          return -1;
      } else {
          int index=0;
          for(auto itr = container.begin();itr != container.end();){
              if(*itr == item){
                  break;
              } else {
                  itr++;
                  index++;
              }
          }
          return index;
      }
  };

  • 當元素型別為自定義的class或struct時：

  // user defined value type
  template <class Container, class ItemType>
  int isContain(Container const &container, ItemType const &item, bool func(ItemType const, ItemType const)){
      using type = typename std::decay<typename Container::value_type>::type;
      bool isSame = std::is_same<type, typename std::decay<ItemType>::type>::value;
      if(!isSame){
          cout << "error type not same " << endl;
          return -1;
      } else {
          int index=0;
          for(auto itr = container.begin();itr != container.end();){
              if(func(*itr, item)){
                  break;
              } else {
                  itr++;
                  index++;
              }
          }
          return index;
      }
  };

• 對STL容器的元素求和：

  元素為基本型別時，用std::accumulate(container.begin(), container.end(), 0);即可，

  當元素為自定義的型別時：

  template <typename Container, typename ItemType>
  ItemType accumulate(Container container, ItemType getValue){
    ItemType sum;
    for (int i = 0; i < container.size(); ++i) {
        sum += getValue(container.at(i));
    }
    return sum;
  }

  struct Pt{
  int x;
  int y;
  }
  
  int getValue(Pt pt){ return pt.x;}
  
  int main(){
  vector<Pt> pts = {{1,2}, {2,3}, {3,4}};
  auto sum = accumulate(pts,getValue); // 執行OK
  return 0;
  }
  

  這樣寫模板函式，type ItemType 當作函式的返回型別， 傳入匿名（lambda) 函式時會報錯不能通過編譯
  mismatched types ... main()::<lambda(int)>
  例如：

  auto getValueX = [](Pt pt)->int{return pt.x;};
  auto sum = accumulate(pts,getValueX);

  或者：

  auto sum = accumulate(pts,[](Pt pt){return pt.x;});

  所以模板函式傳入的函式應該是函式指標型別， 返回型別用std::result_of獲取：

  template<typename Container, typename Func>
  auto add(Container& container, Func getValue) -> typename std::result_of<Func(typename Container::value_type)>::type {
    using type = typename std::result_of<Func(typename Container::value_type)>::type;
    type sum  = getValue(container.at(0));
    for (int i = 0; i < container.size(); ++i) {
        sum += getValue(container.at(i));
    }
    return sum;
  }