.. highlight:: c++
  :linenothreshold: 5

.. _ref-in-class:

******************************************
Reference-typed Instance Variable in Class
******************************************

TR;DR
=====
+ Reference-typed instance variable must be initialized in the constructor
  using initializer list syntax!
+ Will never be reassigned
+ Later access/modification will only affect the variable bound to it rather
  than the reference itself

Overview
========
+ To be able to refer to an external object without copying it
+ It must be passed to the constructor of the class
+ Reference-typed instance variable must be initialized in the constructor
  using initializer list syntax

.. _ref-in-class-example:

Example
=======

::

  // ==== Reference type in class ====

  // Sharable configuration class
  class Config {
    int value;
   public:
    Config(int value) : config(value) {}  // constructor
    int getValue() { return value; }
  };

  // ---- correct ----
  // Class that need to keep a reference to the configuration
  class MyClass {
    Config &conf;
   public:
    explicit MyClass(Config &conf) : conf(conf) {}
    // other methods can now use "conf" as if it's a Config object
  };

  // ---- Usage ----
  Config config(10);
  MyClass myClass(config);  // so myClass.conf is bound to config

  // ---- wrong ----
  class MyClass {
    Config &conf;
   public:
    explicit MyClass(Config &conf) {
      this->conf = conf;
    }
  };