class constexpr expressions in initializer lists

Question

The following code fails to link with g++ 4.8.2:

#include <map>

struct Foo
{
    constexpr static int foo = 1;
};

static std::map<int, int> map {{1, Foo::foo}};

int main()
{
    return Foo::foo;
}

I get the following error:

g++ -std=c++11 -o foo foo.cc
/tmp/ccZXCwiK.o: In function `__static_initialization_and_destruction_0(int, int)':
foo.cc:(.text+0x51): undefined reference to `Foo::foo'

If I comment out the map, things link just fine. Is this a compiler bug, or is it some corner case I am missing in the standard?

@LightnessRacesinOrbit coliru.stacked-crooked.com/a/d9da4dacc301287d — Johannes Schaub - litb
– Johannes Schaub - litb, Commented Dec 12, 2013 at 18:09
And again, a + magically fixes things: static std::map<int, int> map {{1, +Foo::foo}}; works fine. — dyp
– dyp, Commented Dec 12, 2013 at 18:43
@DyP : Wow. That's quite amazing and quite frightening. I would never put that in my code because the comment I would have to add explaining why it was there would be worse than the "fix." — md5i
– md5i, Commented Dec 12, 2013 at 21:36

Lightness Races in Orbit · Accepted Answer · 2013-12-12 18:11:11Z

7

You forgot to define the static member.

As soon as you odr-use it, a full definition is required. Yes, even though it has an inline initialiser and, yes, even though it's marked constexpr.

Add:

constexpr int Foo::foo;

(example)

Your original code works for me in GCC 4.8.1 via both ideone.com and Coliru, but that's with optimisations turned on (so that constant is likely being supplanted for the symbol in each case). With optimisations off, the error is reproducible.

edited Dec 12, 2013 at 18:11

answered Dec 12, 2013 at 18:06

Lightness Races in Orbit

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1 Comment

md5i Over a year ago

Interesting. This came up in an instance where I was replacing a macro with a constexpr. Figuring out where odr-use applies is definitely causing me to slip up.

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