Template notes

template_class_friend.cpp

// 1, 类内声明同时定义
template <typename T>
class VectorWrapper {
public:
  VectorWrapper(const std::initializer_list<T> &list) { mVector = list; }
  friend std::ostream &operator<<(std::ostream &os, const VectorWrapper<T> &vec_w) {
    vec_w.print_l(os);
    return os;
  }
  
private:
  void print_l(std::ostream &os) const {
    for (const auto &v : mVector)
       std::cout << v << ", ";
  }
  std::vector<T> mVector;  
};

// 2, 类内声明，类外定义（使用不同的模板类名，防止冲突隐藏）
template <typename T>
class VectorWrapper {
public:
  VectorWrapper(const std::initializer_list<T> &list) { mVector = list; }
  // 声明
  template <typename U>
  friend std::ostream &operator<<(std::ostream &os, const VectorWrapper<U> &vec_w);
  
private:
  void print_l(std::ostream &os) const {
    for (const auto &v : mVector)
       std::cout << v << ", ";
  }
  std::vector<T> mVector;  
};

// 定义
template <typename U>
friend std::ostream &operator<<(std::ostream &os, const VectorWrapper<U> &vec_w) {
  vec_w.print_l(os);
  return os;
}

// 3, 类外声明定义，类内实例化（需要前置声明类）
template <typename T>
class VectorWrapper; // forward declaration for friend function to use

// declaration and definition of friend function
template <typename T>
friend std::ostream &operator<<(std::ostream &os, const VectorWrapper<T> &vec_w) {
  vec_w.print_l(os);
  return os;
}

template <typename T>
class VectorWrapper {
public:
  VectorWrapper(const std::initializer_list<T> &list) { mVector = list; }
  // 为类型 T 实例化
  friend std::ostream &operator<<<T>(std::ostream &os, const VectorWrapper<T> &vec_w);
  
private:
  void print_l(std::ostream &os) const {
    for (const auto &v : mVector)
       std::cout << v << ", ";
  }
  std::vector<T> mVector;  
};

template_deduction_guide.cpp

template <typename T>
struct TestString {
    T data;
    TestString(const T &d) : data(d) {}
};

TestString(const char *)->TestString<std::string>; // (1). 推导指引，since C++ 17

int main() {
  // without (1), 无法编译，类型 T 会被推导为 char[6]
  TestString ts{"hello"};
}

/**
 * 传递数组或字符串字面值给模板时要各位小心，如果模板参数此时定义为引用，那么参数就不会退化（decay）。
 * 如 const char[N] 不会隐式转换为 const char *
 */