Reference initialization in C++11 default constructor

Question

struct X {};

struct Y { 
    Y() = default;
    X& x;
};

works fine in C++11. I wish to know how Y::x is actually initialized behind the scenes?

That can't possibly compile. A trivial default constructor default-initializes all members, but references cannot be default-initialized. — Kerrek SB
– Kerrek SB, Commented Apr 18, 2013 at 7:07
@KerrekSB: So I guess it boils down to a quality of implementation thing, let's hope compilers catch up to diagnose this sooner rather than later! — Matthieu M.
– Matthieu M., Commented Apr 18, 2013 at 7:09
@MatthieuM. its a matter of taste if this should be diagnosed. default according to the standard means "do whatever a generated default constructor would be - (even if that means implicit deletion)". However, many compilers warn about the implicit deletion and imo should continue to do so if the constructor is defaulted explicitly. — Arne Mertz
– Arne Mertz, Commented Apr 18, 2013 at 7:16
@KerrekSB it compiles because as in C++03, even explicitly defaulted ctors are generated only if they are actually called. — Arne Mertz
– Arne Mertz, Commented Apr 18, 2013 at 7:17

Michael Burr · Accepted Answer · 2013-04-18 07:24:07Z

13

Even though you explicitly indicate that Y() should be defaulted, the compiler is obligated to delete the default constructor under certain conditions (emphasis added):

8.4.2/4 Explicitly-defaulted functions

Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them (12.1 12.4, 12.8), which might mean defining them as deleted

and

12.1/5 Constructors:

...

... A defaulted default constructor for class X is defined as deleted if:

any non-static data member with no brace-or-equal-initializer is of reference type

But it's not an error to define a deleted function or constructor unless you actually try to use it:

8.4.3/2 Deleted defintions

A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed.

answered Apr 18, 2013 at 7:24

Michael Burr

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Kerrek SB Over a year ago

An important additional point is in my opinion 12.1/6: the defaulted constructor is defined if and when it is ODR-used, but not otherwise.

ForEveR · Accepted Answer · 2013-04-18 07:35:26Z

11

It doesn't compile in any major compiler. It will compile, until of creation of object of type Y.

If you create object of type Y, output of clang will be

error: call to implicitly-deleted default constructor of 'Y'
note: explicitly defaulted function was implicitly deleted here
    Y() = default;
note: default constructor of 'Y' is implicitly deleted because field
'x' of reference type 'X &' would not be initialized
    X& x;

When you declare user-defined constructor, which is simply empty function, there is error, without creation of object.

Michael Burr is right. An implicitly-defaulted constructor works perfectly fine. There is no problems with diagnostic here, as I can see.

edited Apr 18, 2013 at 7:35

answered Apr 18, 2013 at 7:03

ForEveR

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Comments

Rapptz · Accepted Answer · 2013-04-18 07:05:01Z

8

It "works" if you don't make a Y object. Once you make one you will get an error:

(GCC 4.8.0)

error: uninitialized reference member in 'struct Y'

answered Apr 18, 2013 at 7:05

Rapptz

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