/* Code for exercise 10.12. |  | "C++ Solutions--Companion to The C++ Programming Language, Third Edition" | by David Vandevoorde; Addison-Wesley 1998; ISBN 0-201-30965-3. | | Permission to use, copy, modify, distribute and sell this software | and its documentation for any purpose is hereby granted without fee. | The author makes no representations about the suitability of this | software for any purpose.  It is provided "as is" without express or | implied warranty. `----------------------------------------------------------------------*/#include <iostream>#include <typeinfo>/* The following can go in a header file */struct Char_queue {   Char_queue(unsigned capacity = 0): in_(&out_cell_)      { out_cell_.next_ = 0; }   ~Char_queue();   bool empty() const { return in_==&out_cell_; }   char dequeue();   void enqueue(char);   bool full() const { return false; }   static bool const fixed_capacity = false;private:   struct Cell {      Cell *next_;      char c_;   };   Cell out_cell_;   Cell *in_;};/* Header file would stop here. *//* Now the implementation file */#include <stdexcept>#include <string>#include <new>Char_queue::~Char_queue() {   // Delete all elements in the queue:   for (Cell *p = out_cell_.next_; p;) {         Cell *victim = p;         p = p->next_;      delete victim;   }}char Char_queue::dequeue() {   if (!empty()) {      Cell *victim = out_cell_.next_;      if (victim==in_) { in_ = &out_cell_; }      char c = victim->c_;      out_cell_.next_ = victim->next_;      delete victim;      return c;   } else      throw std::underflow_error(std::string("queue"));}void Char_queue::enqueue(char c) {   // the 'std::nothrow' prevents the bad_alloc exception:   in_->next_ = new(std::nothrow) Cell;   if (!in_->next_)      throw std::overflow_error(std::string("queue"));   in_ = in_->next_;   in_->next_ = 0;   in_->c_ = c;}/* A brief test of the above implementation: */int main() {   Char_queue q;   while (1) {      if (q.empty()) { std::cout << "Queue is empty.\n"; }      else if (q.full()) { std::cout << "Queue is full.\n"; }      char cmd, ch;      std::cin >> cmd;      try {         switch (cmd) {         case 'e': case 'E':            std::cin >> ch;            q.enqueue(ch);            break;         case 'd': case 'D':            std::cout << "Dequeued " << q.dequeue() << '\n';            break;         case 'q': case 'Q':            std::cout << "Quitting!\n";            return 0;         default:            std::cerr << "Invalid command!\n";         }      } catch (std::exception &x) {         std::cerr << "Caught exception " << typeid(x).name()                   << " (" << x.what() << ")\n";      }   }   return 0;}