Difference between getters ending with const and const&

Question

I have watched a talk (exact timestamp, not explained by him) by Nicolai Josuttis (member of C++ standard committee) and he stated that getters should be written like so:

const std::string& getName() const&
{
     return memberStringVar;
}

Ever since C++11. The question is, what is the difference comparing to this getter?

const std::string& getName() const
{
     return memberStringVar;
}

@PasserBy can you explain why this does not work for an rvalue? — Michael Kemmerzell, Feb 22 '19 at 07:01
@PasserBy The first one compiles with clang with your example. — Croolman, Feb 22 '19 at 07:13
Apparently I misunderstood the ref-qualifier, I have no idea why things are the way they are. — Passer By, Feb 22 '19 at 07:18
In the talk, there is a `&&` getter (which is for rvalues). If this getter exists, the other must have ref-qualifier, so your second example cannot work. — geza, Feb 22 '19 at 07:31
Possible duplicate of [What is "rvalue reference for \*this"?](https://stackoverflow.com/questions/8610571/what-is-rvalue-reference-for-this) — , Feb 22 '19 at 07:42
@Pi you might be right, although it is not as straightfoward. — Croolman, Feb 22 '19 at 08:06
Diskussed in https://meta.stackoverflow.com/questions/381648/triage-post-states-it-is-a-duplicate-but-is-considered-ok?cb=1 as triage fail. migh catch some up/dv — Patrick Artner, Mar 23 '19 at 18:28

P.W · Accepted Answer · 2019-02-25T09:31:05.300

In the example given in the talk, there are two overloads of getName() given. One with the && and the other with the const& qualifiers.

Without the & after the const, the function const std::string& getName() const cannot be overloaded with the overload for rvalues string Customer::getName() &&.

You would then have to remove the rvalue overload from the code completely if you want it to work.

Since ref qualified member functions were added only in C++11 (making the getter for rvalues possible), the change from const std::string& getName() const to const std::string& getName() const& was needed to make both overloads possible.

The C++17 standard draft n4659 states :

16.1 Overloadable declarations [over.load]
...

2 Certain function declarations cannot be overloaded:
...
(2.3) — Member function declarations with the same name and the same parameter-type-list as well as member function template declarations with the same name, the same parameter-type-list, and the same template parameter lists cannot be overloaded if any of them, but not all, have a ref-qualifier.

Since there is one overload of getName() with a ref-qualifier (&&), the other one also should have the ref qualifier. This is why const& is required.

Difference between getters ending with const and const&

1 Answers1