Assignment to non-const reference

Question

VS2015 Update 3 compiles this with no error and no warning. Yet I was under the impression that temporaries could only be bound to const references. Is this non-conformant or am I misunderstanding something?

struct Foo {};
Foo Func6() { return Foo(); }

TEST(Arguments, NonConstReference)
{
   Foo& bob = Func6();
}

EDIT

The question linked by marcinj is the same issue, although that question is not specifically about VS2015 and was not located when I typed this one up.

I had tested using /W4 to see if I got a warning and didn't see one, but testing it again I find that now I do. When opening the project properties to adjust a setting VS2015 has a habit of showing the properties dialog set for a different build configuration than the currently selected one, a most unhelpful behaviour that has caught me out many times, (and will I'm sure continue to do so).

As is mentioned in that other question using Disable Language Extensions (/Za) makes this an error, but is unfortunately not a usable solution as Microsoft's own platform headers won't then compile.

score 8 · Accepted Answer · answered Nov 24 '16 at 15:16

8

The MSVC compiler has a non-standard extension, enabled by default, which allows temporaries to bind to non-const references. To disable this, use the command-line option /Za or the corresponding Project Property.

answered Nov 24 '16 at 15:16

Angew is no longer proud of SO

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Never understood the use of this. – Hatted Rooster Nov 24 '16 at 15:17
@GillBates even more useless when you have rvalue refs! – Guillaume Racicot Nov 24 '16 at 15:18
1

@GuillaumeRacicot Of course, this extension predates rvalue references by at least one decade, so I don't think that's fair comparison. – Angew is no longer proud of SO Nov 24 '16 at 15:20
@Angew although this is the VS2015 compiler we are talking about here, so MS has had plenty of opportunity to fix it. – Neutrino Nov 24 '16 at 16:40
1

Marking this as the answer although it's worth noting that using /Za will cause MS own platform headers to fail to compile, so that's rarely a useful workaround. – Neutrino Nov 24 '16 at 16:41
In VS2017 there is `/permissive-` to disable permissive mode. – Oktalist Nov 24 '16 at 19:30
@Neutrino How would you fix this without breaking **tons** of legacy code? – Angew is no longer proud of SO Nov 24 '16 at 20:04
@Angew Easily. MS should fix their platform headers to not rely on this feature, then the compiler extension that enables this behaviour should be added to a separate 'legacy mode' option that is _not_ on by default. – Neutrino Nov 25 '16 at 09:52

Assignment to non-const reference

1 Answers1