class Common
{
public:
  Common()
  {
    std::cout << "Common::Common" << std::endl;
  }

  Common(const Common &r)
  {
    std::cout << "Common::Common copy-constructor" << std::endl;
  }

  ~Common()
  {
    std::cout << "Common::~Common" << std::endl;
  }
};

class BitCopy
{
public:
  BitCopy()
    : m_p(new Common)
  {
    std::cout << "BitCopy::BitCopy" << std::endl;
  }

  ~BitCopy()
  {
    std::cout << "BitCopy::~BitCopy" << std::endl;
    if (m_p) {
      delete m_p;
      m_p = NULL;
    }
  }

private:
  Common *m_p;
};

int main()
{
  BitCopy a;
  BitCopy b(a);
  return 0;
}
log如下：
Common::Common
BitCopy::BitCopy
BitCopy::~BitCopy
Common::~Common
BitCopy::~BitCopy
Common::~Common
*** Error in `./a.out': double free or corruption (fasttop): 0x0000000001f4e010 ***
已放弃 (核心已转储)

class ValueCopy
{
public:
  ValueCopy()
    : m_p(new Common)
  {
    std::cout << "ValueCopy::ValueCopy" << std::endl;
  }

  ValueCopy(const ValueCopy &r)
    : m_p(new Common(*r.m_p))
  {
    std::cout << "ValueCopy::ValueCopy copy-constructor" << std::endl;
  }

  ~ValueCopy()
  {
    std::cout << "ValueCopy::~ValueCopy" << std::endl;
    if (m_p) {
      delete m_p;
      m_p = NULL;
    }
  }

private:
  Common *m_p;
};

int main()
{
  ValueCopy c;
  ValueCopy d(c);
  return 0;
}
Common::Common
ValueCopy::ValueCopy
Common::Common copy-constructor
ValueCopy::ValueCopy copy-constructor
ValueCopy::~ValueCopy
Common::~Common
ValueCopy::~ValueCopy
Common::~Common

class A
{
public:
  A()
    : m_refCount(1)
  {
    std::cout << "A::A" << std::endl;
  }

  A(const A &r)
    : m_refCount(1)
  {
    std::cout << "A::A copy-constructor" << std::endl;
  }

  ~A()
  {
    std::cout << "A::~A" << std::endl;
  }

  void attach()
  {
    std::cout << "A::attach" << std::endl;
    ++m_refCount;
  }

  void detach()
  {
    if (m_refCount != 0) {
      std::cout << "A::detach " << m_refCount << std::endl;
      if (--m_refCount == 0) {
        delete this;
      }
    }
  }

private:
  int m_refCount;
};

class B
{
public:
  B()
    : m_pA(new A)
  {
    std::cout << "B::B" << std::endl;
  }

  B(const B &r)
    : m_pA(r.m_pA)
  {
    std::cout << "B::B copy-constructor" << std::endl;
    m_pA->attach();
  }

  ~B()
  {
    std::cout << "B::~B" << std::endl;
    m_pA->detach();
  }

private:
  A* m_pA;
};

int main()
{
  B e;
  B f(e);
  return 0;
}
A::A
B::B
B::B copy-constructor
A::attach
B::~B
A::detach 2
B::~B
A::detach 1
A::~A