Intercept C++ implicit copy constructor, or invoke its functionality

Question

Given:

class Foo {

private:
    static int cntFoos;

    //... stuff...

public:
     Foo() { cntFoos++; }
     ~Foo() { cntFoos--; }
};

... where "stuff" may be any set of properties. (The idea is to have a counter of instances of that class)

Then:

Foo aFoo;
Foo twoFoo=aFoo;

Will invoke the automatic copy constructor, and thus I'd miss counting this one.

Is there a way to keep that counter reflecting the new instances created automatically? If I implement the explicit copy constructor, I will have to assign all the properties one by one. However, I want a shallow, memberwise copy. I don't need to perform a deep copy, so it seems like a lot of unnecessary work to implement an explicit copy constructor.

Answer 1

Since you want the default behavior for most members and only need special handling for one (static) member, why not encapsulate that special handling in its own class and make a member variable of that class? Like this:

template<typename T>
class InstanceCounter
{
public:
  static int Count;

  // Automatically invoked when a class containing it is created.
  InstanceCounter() { Count++; }

  // Automatically invoked when a class containing it is destroyed.
  ~InstanceCounter() { Count--; }

  // Automatically invoked when a class containing it is copy-constructed.
  InstanceCounter(const InstanceCounter& rhs) { Count++; }

  // No need to override operator=

  // Allow this counter to be used as an int.    
  operator int() const { return Count; }
};

template<typename T>
int InstanceCounter<T>::Count;

class Foo
{
public:
  InstanceCounter<Foo> count;
};

Implementation notes:

• I made InstanceCounter a template so that different classes can easily have their own instance counts.
• For C++11, you'll also want to provide a move constructor and a move assignment operator for InstanceCounter.

Alternatively, and probably better, using the CRTP idiom:

template<typename T>
class InstanceCounted
{
public:
  static int InstanceCount;

  // Automatically invoked when a class containing it is created.
  InstanceCounted() { InstanceCount++; }

  // Automatically invoked when a class containing it is destroyed.
  ~InstanceCounted() { InstanceCount--; }

  // Automatically invoked when a class containing it is copy-constructed.
  InstanceCounted(const InstanceCounted& rhs) { InstanceCount++; }

  // No need to override operator=
};

template<typename T>
int InstanceCounted<T>::InstanceCount;

class Foo : public InstanceCounted<Foo>
{
  // insert class contents here
};
// Now we can access Foo::InstanceCount.