C++11 Magic Statics Workaround in Visual Studio 2012

Question

I have some objects defined as global, which means their constructor are entered before program start or under DllMain(). This causes some issues, and that is why I want to move them to local static.

Our main project are mainly under Visual Studio 2012. According to the these post.

• “Magic static” singleton crashing when referenced in static destruction phase of another translation unit"
• Support For C++11/14/17 Features (Modern C++)

The Magic Statics is not implemented in Visual Studio 2012 (until VS 2015). So the local static variable initialization is not automatically protected from concurrent access by compiler.

What is a suitable workaround or solution for this case?

---

Here is what I tried:

Add a lock to protect. But if the variable initialized before the function is entered, my lock may be useless...

// data object
struct Cat
{
    Cat()
    {
        std::cout << __FUNCTION__ <<  "\n";
    }
};

// lock mutex for singleton getter
static std::mutex getcat_m;

// singleton getter
Cat& GetCat(){
    std::cout << __FUNCTION__ <<  " In\n";

    std::lock_guard<std::mutex> lk(getcat_m);
    static Cat cat;

    std::cout << __FUNCTION__ <<  " Out\n";
    return cat;
}

int main(int argc, char* argv[])
{
    std::cout << __FUNCTION__ <<  " In\n";
    Cat& cat = GetCat();
    std::cout << __FUNCTION__ <<  " Out\n";

    std::cin.ignore();
    return 0;
}

It actually shows as expected. The constructor is called after function is entered.

main In
GetCat In
Cat::Cat
GetCat Out
main Out

But I am not sure whether this is a suitable solution.

Answer 1

Thanks to A. A in comment for the link 18.2.4 Use std::call_once rather than the Double-Checked Locking pattern.

I referenced the example from the Guildline, and rewrite my singleton getter code as the following.

// Cat.cpp
static std::once_flag once_Cat;
static Cat* gpCat = nullptr;
void init_Cat() { gpCat = new Cat(); }

Cat& GetCat() {
    std::call_once(once_Cat, init_Cat);
    return *gpCat;
}