Is it a good idea to always return references for member variable getters?

问题

If I have a class that has many int, float, and enum member variables, is it considered efficient and/or good practice to return them as references rather than copies, and return constant references where no changes should be made? Or is there a reason I should return them as copies?

回答1:

There is no reason to return primitive types such as int and float by reference, unless you want to allow them to be changed. Returning them by reference is actually less efficient because it saves nothing (ints and pointers are usually the same size) while the dereferencing actually adds overhead.

回答2:

If they are constant references, maybe it is OK. If they are not constant references, probably not.

As to efficiency - on a 64-bit machine, the references will be 64-bit quantities (pointers in disguise); int and float and enum will be smaller. If you return a reference, you are forcing a level of indirection; it is less efficient.

So, especially for built-in types as return values, it is generally better to return the value rather than a reference.

回答3:

Some cases it is necessary:

Look at overloaded operator[] for any class. It usually has two versions. The mutating version has to return a reference.

int &operator[](int index);           // by reference
int operator[](int index) const;      // by value

In general, It is OK to allow access to class members by trusted entities by a class e.g. friends. In case these trusted entities also need to modify the state, references or pointers to the class members, are the only options one has.

In many cases, references usually simplify syntax e.g where 'v' is STL vector.

v.at(1) = 2 vs *(v.at(1)) = 2;

回答4:

This is probably mostly a matter of style or preference. One reason to not return references is because you are using getters and setters to allow you to change the implementation of those members, If you changed a private member to another type, or removed it completely because it can be computed, then you no longer have the ability to return a reference, since there's nothing to reference.

On the other hand, returning references for non-trivial types (compound classes) can speed up your code a bit over making a copy, and you can allow those members to be assigned through the returned reference (if desired).

回答5:

Almost, const references are better. For ints and such theres no point because you would want them to be changed or because they are the same size (or nearly) as a reference.

So yes it is a good idea. I prefer another language or to hack away at my own C++ stuff and just allow the var to be public (once again it just my own stuff)

回答6:

This is a performance question mostly but from a robustness point of view I would say it's preferably to return values instead of const references. The reason being that even const references weakens encapsulation. Consider this:

struct SomeClass
{
   std::vector<int> const & SomeInts () const;
   void AddAnInt (int i);  // Adds an integer to the vector of ints.
private:
   std::vector<int> m_someInts;
};

bool ShouldIAddThisInt(int i);

void F (SomeClass & sc)
{
   auto someInts = sc.SomeInts ();
   auto end = someInts.end ();
   for (auto iter = someInts.begin (); iter != end; ++iter)
   {
      if (ShouldIAddThisInt(*iter))
      {
         // oops invalidates the iterators
         sc.AddAnInt (*iter);
      }
   }  
}

So in case it makes semantically sense and we can avoid excessive dynamic allocations I prefer return by value.

回答7:

Getters are for emissions of a class say Exhaust Car.emit(), where the car has just created the Exhaust.

If you are bound to write const Seat& Car.get_front_seat()
to have later sit in the Driver, you can immediately notice that something is wrong.
Correcly, you'd rather write Car.get_in_driver(Driver)
which then calls directly seat.sit_into(Driver).

This second method easily avoids those awkward situations when you get_front_seat but the door is closed and you virtually push in the driver through the closed door. Remember, you have only asked for a seat! :)

All in all: always return by value (and rely on return value optimization), or realize it is time for changing your design.

The background: classes were created so that data can be coupled together with its accessor functionality, localizing bugs etc. Thus classes are never activity, but data oriented.

Further pitfalls: in c++ if you return something by const ref, then you can easily forget it is only a ref and once your object is destructed you can be left with an invalid ref. Otherwise, that object will be copied once it leaves the getter anyway. But unnecessay copies are avoided by the compiler, see Return Value Optimization.

来源：https://stackoverflow.com/questions/3744635/is-it-a-good-idea-to-always-return-references-for-member-variable-getters