Defer implementation in C++

defer.cpp

#include <iostream>
#include <functional>
#include <stack>

class Deferrer
{
  public:
    Deferrer() {}
    ~Deferrer() { callAll(); }

    void addCall(std::function<void()> &&func)
    {
      _callStack.push(std::forward<decltype(func)>(func));
    }

  private:
    std::stack<std::function<void()>> _callStack;

    void callAll()
    {
       while(!_callStack.empty())
       {
          _callStack.top()();
          _callStack.pop();
       }
    }
};

// We might want perfect forwarding here, but that's in the TODO list for now
#define defer(...) \
  do { \
    auto deferred = std::bind(__VA_ARGS__); \
    __deferrer.addCall(deferred); \
  } while(0);

#define allow_deferred() \
  Deferrer __deferrer;

class A
{
  public:
    A() {}
    ~A() { std::cout << "Finishing A..." << std::endl; }
    void f() { std::cout << "Inside A::f" << std::endl; }
};

void func(int &val)
{
  A a;
  // the allow_deferred call should be after all local variable declarations
  allow_deferred();

  defer(&A::f, &a);

  val = 1;

  for(int i = 0; i < 4; i++)
  {
    defer([=]() { std::cout << i << std::endl; });
  }

  defer([&val]() { val = 42; });
}

int main()
{
  int i = 0;
  func(i);

  std::cout << "i = " << i << std::endl; 

  return 0;
}