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Reference-typed Instance Variable in Class

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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

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;
  }
};