vector的push_back对于拷贝构造和赋值操作的调用

对应与CArray，补充一个vector的测试。

class A
{
  int *m_a;
  int bb;
private:
  A();
public:

  A(int n)
  {
    m_a = new int;
    *m_a = n;
  }
  A(A const & _a)
  {
    m_a = 0;
    *this = _a;
  }
  ~A()
  {
    delete m_a;
  }
  A &operator=(A const &_a)
  {
    if(this == &_a) return *this;
    if(m_a) delete m_a; 
    m_a = new int;
    *m_a = *_a.m_a;
    return *this;
  }
};

vector<A> arr;
  A a(1);   
  arr.push_back(a);
  int fenge = 10;
  arr.erase(arr.begin());
  fenge = 9;

push_back时生成对象：

0x0012fb48

0x003ba320

两次调用 copy构造，

然后删除第一个临时对象0x0012fb48

erase时调用第二个对象，也就是在容器中对象的析构函数。

如果将vector换成list，则push_back时仅调用一次copy构造。