l13

Шаблони и вложени класове

Шаблони.
Класове-шаблони дават възможност да се използват данни от произволен тип. Вече сме използвали класове-шаблони в:

vector<int> v_i; 
vector<double> v_d;
vector<Employee> v_e;

За да дефинираме клас-шаблон, означаваме произволен тип с T и добавяме template<typename T> преди дефиницията на класа.

Пример. Нека дефинираме наредена двойка елементи с данни от произволен тип:

template<typename T>
class Pair {
public:
   Pair(T a, T b);
   T get_first() const;
   T get_second() const;
private:
   T first;
   T second;
};

Всички член-функции се дефинират също като шаблони.

// pairs.cpp
#include <iostream>
 #include <string>
 using namespace std;

 template<typename T>
 class Pair {
 public:
    Pair(T a, T b);
    T get_first() const;
    T get_second() const;
    void print() const;
 private:
    T first;
    T second;
 };
 
 template<typename T>
 Pair<T>::Pair(T a, T b)
 {  first = a;
    second = b;
 }
 
 template<typename T>
 T Pair<T>::get_first() const
 {  return first; }
 
 template<typename T>
 T Pair<T>::get_second() const
 {  return second; }
 
 template<typename T>
 void Pair<T>::print() const
 { cout << "Pair: (" << first << ","
        << second << ")" << endl;  }

 int main()
 {  
    Pair<int> integers(10,22);
    integers.print();

    Pair<double> doubles(1.5, 2.25);
    doubles.print();

    Pair<string> strings("One", "Two");
    strings.print(); 
    return 0;
 }

Пример. Класът List (list2.cpp,list0.cpp) съхранява свързан списък от низове.
Използвайки шаблони, List ще може да съхранява стойности от произволен тип, както това става в стандартния клас list от STL.
За тази цел дефинираме шаблон на клас, като задаваме формален параметър T на шаблона с конструкцията:

template<typename T> class List;

При създаване на обект от този клас задаваме фактически параметър string на шаблона по познатата схема:

List<string> staff;

Дефиниция на класа шаблон List в сравнение с по-рано дефинирания клас List:

template<typename T>
class List {
public:
   List();
   void push_back(T s);
   void insert(Iterator<T> pos, T s);
   void erase(Iterator<T> pos);
   Iterator<T> begin();
   Iterator<T> end();
private:
   Node<T>* first;
   Node<T>* last;
};

class List {
public:
   List();
   void push_back(string s);
   void insert(Iterator pos, string s);
   Iterator erase(Iterator pos);
   Iterator begin();
   Iterator end();
private:
   Node* first;
   Node* last;
};

Ще пренапишем класовете за свързан списък, като използваме шаблони.

// list.cpp
#include <string> #include <iostream> #include <cassert> using namespace std; /* forward declarations */ template<typename T> class List; template<typename T> class Iterator; /** A class to hold the nodes of the linked list. */ template<typename T> class Node { public: /** Constructs a node for a given data value. @param s the data to store in this node */ Node(T s); private: T data; Node<T>* previous; Node<T>* next; friend class List<T>; friend class Iterator<T>; }; /** An iterator denotes a position in the list or past the end of the list. */ template<typename T> class Iterator { public: /** Constructs an iterator that is not attached to any list. */ Iterator(); /** Looks up the value at a position. @return the value of the Node to which the iterator points */ T operator*() const; /** Advances the iterator to the next position. */ void operator++(int dummy); /** Moves the iterator to the previous position. */ void operator--(int dummy); /** Compares two iterators. @param b the iterator to compare with this iterator @return true if this iterator and b are equal */ bool operator==(Iterator<T> b) const; /** Compares two iterators. @param b the iterator to compare with this iterator @return true if this iterator and b are not equal */ bool operator!=(Iterator<T> b) const; private: Node<T>* position; Node<T>* last; friend class List<T>; }; /**A linked list of values of a given type. @param T the type of the list values */ template<typename T> class List { public: /** Constructs an empty list. */ List(); /** Constructs a list as a copy of another list. @param b the list to copy */ List(const List<T>& b); /** Deletes all nodes of this list. */ ~List(); /** Assigns another list to this list. @param b the list to assign @return a reference to this list */ List<T>& operator=(const List<T>& b); /** Appends an element to the list. @param s the value to append */ void push_back(T s); /** Inserts an element into the list. @param iter the position before which to insert @param s the value to append */ void insert(Iterator<T> iter, T s); /** Removes an element from the list. @param i the position to remove @return an iterator pointing to the element after the erased element */ Iterator<T> erase(Iterator<T> i); /** Gets the beginning position of the list. @return an iterator pointing to the beginning of the list */ Iterator<T> begin() const; /** Gets the past-the-end position of the list. @return an iterator pointing past the end of the list */ Iterator<T> end() const; private: /** Copies another list to this list. @param b the list to copy */ void copy(const List<T>& b); /** Deletes all nodes of this list. */ void free(); Node<T>* first; Node<T>* last; }; template<typename T> List<T>::List() { first = NULL; last = NULL; } template<typename T> List<T>::~List() { free(); } template<typename T> List<T>::List(const List<T>& b) { first = NULL; last = NULL; copy(b); } template<typename T> List<T>& List<T>::operator=(const List<T>& b) { if (this != &b) { free(); copy(b); } return *this; } template<typename T> void List<T>::push_back(T s) { Node<T>* newnode = new Node<T>(s); if (last == NULL) /* list is empty */ { first = newnode; last = newnode; } else { newnode->previous = last; last->next = newnode; last = newnode; } } template<typename T> void List<T>::insert(Iterator<T> iter, T s) { if (iter.position == NULL) { push_back(s); return; } Node<T>* after = iter.position; Node<T>* before = after->previous; Node<T>* newnode = new Node<T>(s); newnode->previous = before; newnode->next = after; after->previous = newnode; if (before == NULL) /* insert at beginning */ first = newnode; else before->next = newnode; } template<typename T> Iterator<T> List<T>::erase(Iterator<T> i) { Iterator<T> iter = i; assert(iter.position != NULL); Node<T>* remove = iter.position; Node<T>* before = remove->previous; Node<T>* after = remove->next; if (remove == first) first = after; else before->next = after; if (remove == last) last = before; else after->previous = before; iter.position = after; delete remove; return iter; } template<typename T> Iterator<T> List<T>::begin() const { Iterator<T> iter; iter.position = first; iter.last = last; return iter; } template<typename T> Iterator<T> List<T>::end() const { Iterator<T> iter; iter.position = NULL; iter.last = last; return iter; } template<typename T> Iterator<T>::Iterator() { position = NULL; last = NULL; } template<typename T> T Iterator<T>::operator*() const { assert(position != NULL); return position->data; } template<typename T> void Iterator<T>::operator++(int dummy) { assert(position != NULL); position = position->next; } template<typename T> void Iterator<T>::operator--(int dummy) { if (position == NULL) position = last; else position = position->previous; assert(position != NULL); } template<typename T> bool Iterator<T>::operator==(Iterator<T> b) const { return position == b.position; } template<typename T> bool Iterator<T>::operator!=(Iterator<T> b) const { return position != b.position; } template<typename T> Node<T>::Node(T s) { data = s; previous = NULL; next = NULL; } template<typename T> void List<T>::copy(const List<T>& b) { for (Iterator<T> p = b.begin(); p != b.end(); p++) push_back(*p); } template<typename T> void List<T>::free() { while (begin() != end()) erase(begin()); } int main() { List<string> staff; staff.push_back("Cracker, Carl"); staff.push_back("Hacker, Harry"); staff.push_back("Lam, Larry"); staff.push_back("Sandman, Susan"); /* add a value in fourth place */ Iterator<string> pos; pos = staff.begin(); pos++; pos++; pos++; staff.insert(pos, "Reindeer, Rudolf"); /* remove the value in second place */ pos = staff.begin(); pos++; staff.erase(pos); /* print all values */ for (pos = staff.begin(); pos != staff.end(); pos++) cout << *pos << "\n"; return 0; }

Вложени класове

В STL класът iterator е дефиниран в класа list:

list<string>::iterator pos = staff.begin();

За да се вложи един клас в друг, вътрешният клас се дефинира във външния клас:

class List {
   ...
   class Iterator;
   ...
};

Пример. Клaсът List със същия интерфейс, както и класът list от STL.

// list1.cpp
#include <string>#include <iostream>#include <cassert>using namespace std;template<typename T> class List;template<typename T>class Node {public: Node(T s);private: T data; Node<T>* previous; Node<T>* next;friend class List<T>;friend class List<T>::Iterator;}; template<typename T>class List {public: List(); List(const List<T>& b); ~List(); List<T>& operator=(const List<T>& b); class Iterator; void push_back(T s); void insert(Iterator iter, T s); Iterator erase(Iterator i); Iterator begin(); Iterator end();private: void copy(const List<T>& b); void free(); Node<T>* first; Node<T>* last;};template<typename T>class List<T>::Iterator {public: Iterator(); T operator*() const; void operator++(int dummy); void operator--(int dummy); bool operator==(Iterator b) const; bool operator!=(Iterator b) const;private: Node<T>* position; Node<T>* last;friend class List<T>;};template<typename T>List<T>::List(){ first = NULL; last = NULL;}template<typename T>List<T>::~List(){ free();}template<typename T>List<T>::List(const List<T>& b){ first = NULL; last = NULL; copy(b);}template<typename T>List<T>& List<T>::operator=(const List<T>& b){ if (this != &b) { free(); copy(b); } return *this;}template<typename T>void List<T>::push_back(T s){ Node<T>* newnode = new Node<T>(s); if (last == NULL) /* list is empty */ { first = newnode; last = newnode; } else { newnode->previous = last; last->next = newnode; last = newnode; }}template<typename T>void List<T>::insert(Iterator iter, T s){ if (iter.position == NULL) { push_back(s); return; } Node<T>* after = iter.position; Node<T>* before = after->previous; Node<T>* newnode = new Node<T>(s); newnode->previous = before; newnode->next = after; after->previous = newnode; if (before == NULL) /* insert at beginning */ first = newnode; else before->next = newnode;}template<typename T>typename List<T>::Iterator List<T>::erase(Iterator i){ Iterator iter = i; assert(iter.position != NULL); Node<T>* remove = iter.position; Node<T>* before = remove->previous; Node<T>* after = remove->next; if (remove == first) first = after; else before->next = after; if (remove == last) last = before; else after->previous = before; iter.position = after; delete remove; return iter;}template<typename T>typename List<T>::Iterator List<T>::begin(){ Iterator iter; iter.position = first; iter.last = last; return iter;}template<typename T>typename List<T>::Iterator List<T>::end(){ Iterator iter; iter.position = NULL; iter.last = last; return iter;}template<typename T>List<T>::Iterator::Iterator(){ position = NULL; last = NULL;}template<typename T>T List<T>::Iterator::operator*() const{ assert(position != NULL); return position->data;}template<typename T>void List<T>::Iterator::operator++(int dummy){ assert(position != NULL); position = position->next;}template<typename T>void List<T>::Iterator::operator--(int dummy){ if (position == NULL) position = last; else position = position->previous; assert(position != NULL);}template<typename T>bool List<T>::Iterator::operator==(Iterator b) const{ return position == b.position;}template<typename T>bool List<T>::Iterator::operator!=(Iterator b) const{ return position != b.position;}template<typename T>Node<T>::Node(T s){ data = s; previous = NULL; next = NULL;}template<typename T>void List<T>::copy(const List<T>& b){ for (Iterator p = b.begin(); p != b.end(); p++) push_back(*p);}template<typename T>void List<T>::free(){ while (begin() != end()) erase(begin());}int main(){ List<string> staff; staff.push_back("Cracker, Carl"); staff.push_back("Hacker, Harry"); staff.push_back("Lam, Larry"); staff.push_back("Sandman, Susan"); /* add a value in fourth place */ List<string>::Iterator pos; pos = staff.begin(); pos++; pos++; pos++; staff.insert(pos, "Reindeer, Rudolf"); /* remove the value in second place */ pos = staff.begin(); pos++; staff.erase(pos); /* print all values */ for (pos = staff.begin(); pos != staff.end(); pos++) cout << *pos << "\n"; return 0;}