Should I prefer pointers or references in member data?


Question

This is a simplified example to illustrate the question:

class A {};

class B
{
    B(A& a) : a(a) {}
    A& a;
};

class C
{
    C() : b(a) {} 
    A a;
    B b; 
};

So B is responsible for updating a part of C. I ran the code through lint and it whinged about the reference member: lint#1725. This talks about taking care over default copy and assignments which is fair enough, but default copy and assignment is also bad with pointers, so there's little advantage there.

I always try to use references where I can since naked pointers introduce uncertaintly about who is responsible for deleting that pointer. I prefer to embed objects by value but if I need a pointer, I use auto_ptr in the member data of the class that owns the pointer, and pass the object around as a reference.

I would generally only use a pointer in member data when the pointer could be null or could change. Are there any other reasons to prefer pointers over references for data members?

Is it true to say that an object containing a reference should not be assignable, since a reference should not be changed once initialised?

1
126
4/17/2012 4:51:53 PM

Accepted Answer

Avoid reference members, because they restrict what the implementation of a class can do (including, as you mention, preventing the implementation of an assignment operator) and provide no benefits to what the class can provide.

Example problems:

  • you are forced to initialise the reference in each constructor's initialiser list: there's no way to factor out this initialisation into another function (until C++0x, anyway edit: C++ now has delegating constructors)
  • the reference cannot be rebound or be null. This can be an advantage, but if the code ever needs changing to allow rebinding or for the member to be null, all uses of the member need to change
  • unlike pointer members, references can't easily be replaced by smart pointers or iterators as refactoring might require
  • Whenever a reference is used it looks like value type (. operator etc), but behaves like a pointer (can dangle) - so e.g. Google Style Guide discourages it
51
8/21/2017 1:51:38 PM

My own rule of thumb :

  • Use a reference member when you want the life of your object to be dependent on the life of other objects : it's an explicit way to say that you don't allow the object to be alive without a valid instance of another class - because of no assignment and the obligation to get the references initialization via the constructor. It's a good way to design your class without assuming anything about it's instance being member or not of another class. You only assume that their lives are directly linked to other instances. It allows you to change later how you use your class instance (with new, as a local instance, as a class member, generated by a memory pool in a manager, etc.)
  • Use pointer in other cases : When you want the member to be changed later, use a pointer or a const pointer to be sure to only read the pointed instance. If that type is supposed to be copyable, you cannot use references anyway. Sometimes you also need to initialize the member after a special function call ( init() for example) and then you simply have no choice but to use a pointer. BUT : use asserts in all your member function to quickly detect wrong pointer state!
  • In cases where you want the object lifetime to be dependent on an external object's lifetime, and you also need that type to be copyable, then use pointer members but reference argument in constructor That way you are indicating on construction that the lifetime of this object depends on the argument's lifetime BUT the implementation use pointers to still be copyable. As long as these members are only changed by copy, and your type don't have a default constructor, the type should fullfil both goals.

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow
Icon