template counter : template class « template « C++ Tutorial

1. C++ Tutorial
2. template
3. template class

/* The following code example is taken from the book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */

#include <iostream>

#include <stddef.h>

template <typename CountedType>
class ObjectCounter {
  private:
    static size_t count;    // number of existing objects

  protected:
    // default constructor
    ObjectCounter() {
        ++count;
    }

    // copy constructor
    ObjectCounter (ObjectCounter<CountedType> const&) {
        ++count;
    }

    // destructor
    ~ObjectCounter() {
        --count;
    }

  public:
    // return number of existing objects:
    static size_t live() {
        return count;
    }
};

// initialize counter with zero
template <typename CountedType>
size_t ObjectCounter<CountedType>::count = 0;


template <typename CharT>
class MyString : public ObjectCounter<MyString<CharT> > {
  //...
};

int main()
{
    MyString<char> s1, s2;
    MyString<wchar_t> ws;

    std::cout << "number of MyString<char>:    "
              << MyString<char>::live() << std::endl;
    std::cout << "number of MyString<wchar_t>: "
              << ws.live() << std::endl;
}

number of MyString:    2
number of MyString: 1