MFC Programmer's SourceBook : Thinking in C++

Cleaning up

containers of pointers

In Stlshape.cpp, the pointers did not clean themselves up automatically. It would be convenient to be able to do this easily, rather than writing out the code each time. Here is a function template that will clean up the pointers in any sequence container; note that it is placed in the book’s root directory for easy access:

//: :purge.h
// Delete pointers in an STL sequence container
#ifndef PURGE_H
#define PURGE_H
#include <algorithm>

template<class Seq> void purge(Seq& c) {
  typename Seq::iterator i;
  for(i = c.begin(); i != c.end(); i++) {
    delete *i;
    *i = 0;
  }
}

// Iterator version:
template<class InpIt>
void purge(InpIt begin, InpIt end) {
  while(begin != end) {
    delete *begin;
    *begin = 0;
    begin++;
  }
}
#endif // PURGE_H ///:~

In the first version of purge( ), note that typename is absolutely necessary; indeed this is exactly the case that the keyword was added for: Seq is a template argument, and iterator is something that is nested within that template. So what does Seq::iterator refer to? The typename keyword specifies that it refers to a type, and not something else.

While the container version of purge must work with an STL-style container, the iterator version of purge( ) will work with any range, including an array.

Here is Stlshape.cpp, modified to use the purge( ) function:

//: C20:Stlshape2.cpp
// Stlshape.cpp with the purge() function
#include <vector>
#include <iostream>
#include "../purge.h"
using namespace std;

class Shape {
public:
  virtual void draw() = 0;
  virtual ~Shape() {};
};

class Circle : public Shape {
public:
  void draw() { cout << "Circle::draw\n"; }
  ~Circle() { cout << "~Circle\n"; }
};

class Triangle : public Shape {
public:
  void draw() { cout << "Triangle::draw\n"; }
  ~Triangle() { cout << "~Triangle\n"; }
};

class Square : public Shape {
public:
  void draw() { cout << "Square::draw\n"; }
  ~Square() { cout << "~Square\n"; }
};

typedef std::vector<Shape*> Container;
typedef Container::iterator Iter;

int main() {
  Container shapes;
  shapes.push_back(new Circle);
  shapes.push_back(new Square);
  shapes.push_back(new Triangle);
  for(Iter i = shapes.begin();
      i != shapes.end(); i++)
    (*i)->draw();
  purge(shapes);
} ///:~

When using purge( ), you must be careful to consider ownership issues – if an object pointer is held in more than one container, then you must be sure not to delete it twice, and you don’t want to destroy the object in the first container before the second one is finished with it. Purging the same container twice is not a problem, because purge( ) sets the pointer to zero once it deletes that pointer, and calling delete for a zero pointer is a safe operation.