1.1.9.1 reference basic

• Reference has two characteristics, no null, no change
  1. Not null, when there is a reference, it must refer to an object. So a reference doesn’t need to be test if it’s nullptr, different with pointer.
     if(pointer) // don’t need to do it for a reference. 
        cout<< *pointer<<endl;
  2. No change, a reference always refers to the object with which it’s initialized.
     string &rs = s1; 
     rs = s2; // s1’s value is modified, rs still refer to s1.
• A const object reference passed into a function, if you want to return it , it must be const too.
  //method 1 
  const int &fun(const int& i){ 
    return i; 
  } 
   
  //method 2 
  int &fun(const int& i){ 
    return const_cast<int&>(i); 
  }
• Only const reference can bound to temporary and prolong temporary variable life. 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. 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: 
  f(1); //1 -->"temp obj build by ctor" -->crf
• const reference bound to rvalue used in copy ctor widely. See example below: But in C++11, 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, compiler will bark