1.1.8.1 reference

• Reference has two characteristics, no null, no change
• No null reference, when there is reference, They must refer to an object. So reference don’t need to be test if it’s null, different with pointer.
  if(pointer) // don’t need to do it for a reference. 
     cout<< *pointer<<endl;
• No change reference, a reference always refers to the object with which it’s initialized.
  string &rs = s1; 
  rs = s2; // s1’s value is modified, but rs still refer to s1.
• Some overload operator you should return reference.Because they can be put on the left side of =
  operator =   // a=b=c; 
  operator []  //a[2] = b; 
  operator <<  //cout<<a<<b;
• Prefer to pass or return reference.It will avoid extra copy constructor calling and more efficient. But if you create a obj inside of a function, you cannot return reference of it. You have to return value of it. see effective C++ item 22 and 23.
• A const object reference passed into a function, if you want to return it , it must be const too.
• Only const reference can bound to temporary. temporary is not lvalue, and only lvalue can bound to reference to non-const. And only stack-base const reference can work in this way. If a const reference is class member, it can’t work.
• A const reference can prolong temporary variable. See two examples below:
  //case 1: 
  Foo f(){ 
  return obj; 
  } 
  const Foo & rf = f(); 
   
  //case 2: 
  class Foo(int i); // ctor 
  f(const Foo & crf); 
  //here, must const & in f argument. 
  //if you skip const, compiler will report error: 
  //non const reference from rvalue 
  f(1); 
  //1 -->"temp obj build by ctor" -->crf
• Another const reference example is copy ctor: For a class with a lot allocated resource, please use move copy ctor which explained in the next section:
  class Foo{ 
  Foo(const Foo & foo); 
  } 
  Foo f3 = f1+f2; 
  // without const in copy ctor 
  // it will compile error.