Where does one initialize a Reference member of a class?

Answer 1

References must be initialised from the initialisation list of all the class constructors. This is not unusual, because all instance member variables, particularly embedded members, whether references or not, should always be initialised from the initialisation list in order to ensure the most efficient construction. The initialisation list also allows you to invoke specific base class constructors (where required), rather than the default base class constructor that would be called implicitly.

Note that you cannot initialise a reference with a default value (not even NULL), thus you cannot declare any constructor that does not implicitly pass the required references. In particular, this means you must not declare a default constructor (the compiler-generated version is not generated so long as you declare at least one other constructor). You must also explicitly disable the compiler-generated assignment operator because references cannot be re-assigned once instantiated. You disable this method by declaring it private, but without providing an implementation.

The following stripped-down example shows how to initialise a class that contains two member references:

class foo1 { /*...*/ };

class foo2 { /*...*/ }; // definitions omitted for brevity

class bar {

private:

// member references:

foo1& m_foo1;

foo2& m_foo2;

public:

// ctor and copy ctor with initialisation lists:

bar(foo1& f1, foo2& f2):m_foo1(f1), m_foo2(f2) {}

bar(const bar& b): m_foo1(bar.m_foo1), m_foo2(bar.m_foo2) {}

private:

// assignment operator (disabled):

bar& operator= (const bar&);

};

Note that the assignment operator is declared but is not implemented. Do not include curly braces in the declaration otherwise the method is not disabled and the compiler will complain about the lack of a return value in the implementation.

Note also that definitions can also be split from declarations as required,by defining the initialisation list in the source file rather than the header, like so:

// file: bar.hpp

class bar {

private:

// member references:

foo1& m_foo1;

foo2& m_foo2;

public:

// ctor and copy ctor:

bar(foo1& f1, foo2& f2);

bar(const bar& b);

private:

// assignment operator disabled:

bar& operator= (const bar&);

};

// file: bar.cpp

#include "bar.hpp"

bar::bar(foo1& f1, foo2& f2):m_foo1(f1), m_foo2(f2) {}

bar::bar(const bar& b): m_foo1(bar.m_foo1), m_foo2(bar.m_foo2) {}