References as C++ class members

March 16, 2024  [c++]  [programming] 

It is not an uncommon, although frowned upon, practice to have references or const references as class members in C++. If there is a long-living object you want your class to have associacion with and you are sure its lifespan is by design greater than the your object using it, why not having it as reference?

class Thingy
    // public members

    const UsefulService & service_;
    // other private members

Although it might seem like a totally sensible idea, such design is generally considered bad. There is a specific rule in the C++ Core Guidelines that advices against that:

C.12: Don’t make data members const or references in a copyable or movable type

The primary objection is of course the requirement for the objects to be copyable/moveable. Imagine the Thingy class in the example above: it would be messy if you wanted to have several “thingies” with copying and moving involved. The issues include the following:

In this GitHub issue Bjarne Stroustrup voices an even stronger opinion:

I think we need a rule banning reference members.

As a proper solution, it is recommended to use pointers, either raw or smart, or std::reference_wrapper (emulates a references while being copy-constructible and copy-assignable, see example here). This gives more control over ownership and lifetime of the referred objects.

An alternative opinion is that there can be situations when it is perfectly fine to have reference members. Let’s say we have only one Thingy in our system, without any need for copying or moving, and the instance of UsefulService we refer to has a longer lifetime that the instance of Thingy by design. It is interesting to read the discussion here with both pros and cons on this matter.

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