c++17: header only: class static variable errors

Question

Since C++17, I've been experimenting easier ways to get class static variables. I'm writing a header-only library. Apparently the new meaning of inline for variables is suited for this.

class thingy {
    static inline reporter rep;
};

But I've been getting runtime errors.

I'm using Visual Studio 15.6.4

To test, the following:

• thingy has a static member variable
• The member tells you when it's constructed / destructed and at what address
• Should be constructed and destructed exactly once
• Is #included in two .cpp files

foo.h

#pragma once

#include <iostream>

using namespace std;

struct reporter {
    reporter() {
        cout << "reporter() - " << this << endl;
    }
    ~reporter() {
        cout << "~reporter() - " << this << endl;
    }
};

class thingy {
    static inline reporter rep;
};

main.cpp

#include "foo.h"
int main() {}

foo.cpp

#include "foo.h"

---

Most disappointingly, it prints:

reporter() - 00007FF670E47C80
reporter() - 00007FF670E47C80
~reporter() - 00007FF670E47C80
~reporter() - 00007FF670E47C80

As you can see, it is constructed twice and destructed twice at the same location - not good.

Am I misunderstanding what inline on variables is for?

Is there another way to get class statics in header only? Has this changed in C++17?

Answer 1

It looks like a bug in VS2017.

A few relevant bug reports can be found, although they do not exactly meet your case:

• VC2017 15.5.2 inline variable crashed
• Local static variable initialized multiple times

This will be addressed in 15.7 - thanks for the report! Combining multiple guards for adjacent static variables into a single guard is a backend optimization which can go wrong when inlining under certain circumstances. That's basically the issue here.

Hopefully this static variable when inline-ed bug will be fixed soon in their next patch.

Meanwhile, I found that compiling in Release Mode makes your reporter initialized only once as expected, while in Debug Mode this bug occurs for their backend optimization.

So I guess this won't be going into your products, at least.

Answer 2

It's a bug in MSVC.

The static class member thingy::rep, like thingy, has external linkage, the inline specifier doesn't change that.

As such, there can be only one instance of it in the whole program, and as such it can only be initialized once.

[class.static.data]/5:

Static data members of a class in namespace scope have the linkage of that class.