Reference to a static, function local variable

Question

Is the following code considered safe?

Foo& GetFoo()
{
    static std::once_flag fooInitFlag;
    static Foo f;

    std::call_once(fooInitFlag, [&Foo]()
    {
       Foo.Init();
    });

    return std::ref(f);

}

You don't need the once_flag thing. A single initialization is guaranteed. Use constructor, not Init method- — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented May 21, 2015 at 17:17
Yes. Note you still need that in msvc 2013 but not in C++11 conform compiler where static initialization is thread safe. (do not return std::ref(f), just f) — user2249683
– user2249683, Commented May 21, 2015 at 17:18
You don't need std::ref here; just return f. And it's safe, since f will remain. — Matthew Moss
– Matthew Moss, Commented May 21, 2015 at 17:19

Cheers and hth. - Alf · Accepted Answer · 2015-05-21 17:46:47Z

3

Instead of this posted code:

Foo& GetFoo()
{
    static std::once_flag fooInitFlag;
    static Foo f;

    std::call_once(fooInitFlag, [&Foo]()
    {
       Foo.Init();
    });

    return std::ref(f);
}

do this:

struct Initialized_foo
{
    Foo item;
    Initialized_foo() { item.Init(); }
};

auto get_foo()
    -> Foo&
{
    static Initialized_foo the_foo;
    return the_foo.item;
}

It's not more safe, but it's simpler, and hence more safe against inadvertently introduced bugs.

Note that the standard guarantees a single initialization here, even in the context of multi-threading.

Better, ideally each Foo insteance should be properly initialized by the Foo constructor, not by an Init method. But that's not always possible to arrange.

edited May 21, 2015 at 17:46

answered May 21, 2015 at 17:21

Cheers and hth. - Alf

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4 Comments

user3010322 Over a year ago

You forgot the_foo.item

R Sahu Over a year ago

And static Initialized_foo the_foo;.

Cheers and hth. - Alf Over a year ago

@RSahu: thanks, again. hm. maybe should turn off the TV.

Cheers and hth. - Alf Over a year ago

@tom: I reverted your pure coding style edit. Please don't do that. No need for braces wars and all that.

R Sahu · Accepted Answer · 2015-05-21 17:35:04Z

2

You asked:

Is the following code considered safe?

The answer is yes.

Another implementation:

// Not exposed to the outside.
static Foo& getInitializedFoo();
{
    static Foo f;
    f.Init();
    return f;
}

Foo& GetFoo()
{
    static Foo& f = getInitializedFoo();
    return f;
}

edited May 21, 2015 at 17:35

answered May 21, 2015 at 17:30

R Sahu

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4 Comments

Cheers and hth. - Alf Over a year ago

how about, static Foo& (reference), to avoid the copying. I think it's just a typo.

R Sahu Over a year ago

@Cheersandhth.-Alf, I meant the helper function to return an object by value.

Cheers and hth. - Alf Over a year ago

Well, it would work just as well with a reference (same initialization guarantee), static Foo, and more efficient + less chance of side-effect. Downside that getInitializedFoo then is an internal implementation detail, to only be called once.

R Sahu Over a year ago

@Cheersandhth.-Alf, I see.

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