How do I call a method for all objects in a class?

Question

I want to call a method for all objects in a class at once. What's the simplest way to do that?

Also, how do I call a method for some, but not all objects in a class?

Answer 1

You could do something like this. The object constructor puts the object pointer into a set and the destructor removes them from the set. The set contains pointers to all living objects.

It's pretty self explanatory:

#include <iostream>
#include <set>

class Foo
{
  static std::set<Foo*> FooObjects;    // set of all Foo objects
  int value = 0;
  void Register(Foo*);
  void UnRegister(Foo* f);

 public:
  Foo();
  Foo(int v);
  ~Foo();
  static std::set<Foo*>& FooSet();

  void Display();
  void Multiply(int multiplicator);
};

std::set<Foo*> Foo::FooObjects;

std::set<Foo*>& Foo::FooSet()
{
  return FooObjects;
}

void Foo::Register(Foo* f)
{ // register an object
  FooObjects.insert(f);
}

void Foo::UnRegister(Foo* f)
{
  // unregister an object
  FooObjects.erase(f);
}

Foo::~Foo()
{
  UnRegister(this);
}

Foo::Foo()
{ 
  Register(this);
}

Foo::Foo(int value) : value(value)
{
  Register(this);
}

void Foo::Multiply(int multiplicator)
{
  value *= multiplicator;
}

void Foo::Display()
{
  std::cout << "Foo: " << value << "\n";
}

void SomeFunction()
{
  Foo farray[10];
}

int main()
{
  Foo f1;
  Foo F2(2);
  Foo F3(3);

  SomeFunction();  // 10 Foo objects are created in SomeFunction
                   // but they are destroyed right after and therefore
                   // they never show up below.

  // Display all Foo objects
  for (const auto& object : Foo::FooSet())
    object->Display();

  // Multiply all foo objects by 2
  for (auto& object : Foo::FooSet())
    object->Multiply(2);

  std::cout << "\n";

  // Display all Foo objects
  for (const auto& object : Foo::FooSet())
    object->Display();  
}

Possible output:

Foo: 3
Foo: 2
Foo: 0

Foo: 6
Foo: 4
Foo: 0