undefined reference capturing static variable in lambda

Question

I have a lambda inside of a function that is capturing with [&] and then using a local static variable inside the lambda. I'm not sure if this valid to begin with, but this compiles and links fine:

void Foo()
{
    static int i = 5;

    auto bar = [&]()
    {
        i++;
    };

    bar();
}

int main()
{
    Foo();
}

But by making Foo a templated function:

template <typename T>
void Foo()
{
    static int i = 5;

    auto bar = [&]()
    {
        i++;
    };

    bar();
}

int main()
{
    Foo<int>();
}

I get the following error:

g++-4.7 -std=c++11 main.cpp
/tmp/cctjnzIT.o: In function 'void Foo()::{lambda()#1}::operator()() const':
main.cpp:(.text+0x1a): undefined reference to 'i'
main.cpp:(.text+0x23): undefined reference to 'i'
collect2: error: ld returned 1 exit status

So, I have two questions:

Is using i in the first example even valid c++?
If #1, then what is wrong with the second example? Or is this a gcc bug?

Technically, only variables with automatic storage duration are "captured". In both examples, the reference to i is just an ordinary valid lvalue use. — aschepler
– aschepler, Commented Mar 1, 2013 at 19:22

JaredC · Accepted Answer · 2013-03-02 16:31:38Z

4

1) Yes, you can even drop & from definition because static is always accessible in lambda function. 2) It is bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54276

edited Mar 2, 2013 at 16:31

JaredC

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answered Mar 1, 2013 at 18:16

Yankes

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Comments

Community · Accepted Answer · 2017-05-23 10:25:10Z

-2

1) Yes, assuming you're intending to have 'i''s value persist between calls.

2) It's not a bug in the compiler. The static instance variable needs to also be defined via a template. Please refer to this post.

edited May 23, 2017 at 10:25

CommunityBot

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answered Mar 1, 2013 at 18:18

john.pavan

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2 Comments

JaredC Over a year ago

That post is about static class members, not statics local to functions. How do I explicitly refer to those?

john.pavan Over a year ago

I think you end up with a problem. Mainly b/c each template specialization is instantiated when you use it (and therefore you get multiple instances of your static variable).

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