c++ static unorderd_map used in static method is not initialized

Question

I have some code where a static method is called, and the static std::unordered_map within the same file is not initialized. I understand the static initialization between two compile units is "undefined" and there are many SO questions on the topic; however, when I use an std::vector the issue does not occur. Also, the code can execute, but I am confused as to why these specific compile orders do not work. SO, my questions are:

1. There is another SO question (which I've been unable to find!) about static initialization and dynamic initialization of static variables. Is this error due to std::undored_map actually being a dynamic initialization?
2. is there a way to get this code to initialize the std::unordered_map as I expected? I'm actually trying to create a static library .lib or .a. When I link the static library, it generally needs to come last, and so the error occurs.
3. are there any workarounds for this? One option I've thought of is to create both an std::vector and an std::unordered_map. Use the std::vector while the std::unordered_map is uninitialized (via bool _map_is_initialized). Change the initialization of the std::unordered_map to be explicitly dynamic by calling a function which iterates over the values in the std::vector to produce the std::unordered_map.

Linux

g++ -std=c++1y -g -c thing.cpp
g++ -std=c++1y -g -c main.cpp
g++ -g main.o thing.o -o main
./main

This results in a Floating point exception (core dumped) error. Through gdb, I was able to figure out that hashtable_policy.h trys __num % __den; where __den==0. Also using gdb, it appears as though Thing::Things is uninitialized.

(gdb) break thing.cpp:12
(gdb) run
(gdb) print Thing::Things
No symbol "Things" in specified context.
(gdb) print thing
$1 = (Thing *) 0x618c20

Windows

cl /EHsc /Zi /c main.cpp
cl /EHsc /Zi /c thing.cpp
link /debug main.obj thing.obj
main

In my actual code, this resulted in a very clear segmentation fault; however, this example just opens a popup that says the application failed. ... I have not done better diagnostics.

Code

thing.cpp

#include<iostream>

#include "thing.hpp"

std::vector<Thing*> Before; // EDIT: added

std::unordered_map<std::string, Thing*> Thing::Things;

std::vector<Thing*> After;  // EDIT: added

Thing::Thing(std::string name) : name(name) {

}

bool Thing::Register(Thing *thing) {
    std::cout << "no worries, vectors initialized..." << std::endl;
    Thing::Before.push_back(thing); // EDIT: added
    Thing::After.push_back(thing);  // EDIT: added
    std::cout << "added to vectors, about to fail..." << std::endl;
    Thing::Things[thing->name] = thing;
    return true;
}

thing.hpp

#pragma once
#include <string>
#include <unordered_map>

class Thing {
public:
    static std::vector<Thing*> Before; // EDIT: added

    static std::unordered_map<std::string, Thing*> Things;

    static std::vector<Thing*> After; // EDIT: added

    static bool Register(Thing* thing);

    std::string name;

    Thing(std::string name);
};

#define ADD_THING(thing_name) \
    static bool thing_name## _is_defined = Thing::Register(new Thing( #thing_name ));

main.cpp

#include "thing.hpp"
#include <iostream>

ADD_THING(obligatory);
ADD_THING(foo);
ADD_THING(bar);

int main(int argc, char* argv[]) {
    std::cout << "before loop" << std::endl;
    for (auto thing : Thing::Things) {
        std::cout << "thing.name: " << thing.first << std::endl;
    }
    return 0;
}

EDIT

If the order within a given compile unit is guaranteed, why do static std::vector<Thing*> Thing::Before and static std::vector<Thing*> Thing::After get initialized, but static std::unordered_map<std::string, Thing*> Thing::Things does not?

Answer 1

Static initialization is tricky. As this answer states, the standard provides some guarantees as to the order of initialization within a single "translation unit" (normally a .cpp source file), but none whatsoever concerning what order initializations in different translation units will follow.

When you added the Before and After vectors to the code, you observed that unlike the calls to ordered_map::operator[], the calls to vector::push_back() did not crash the process and concluded that the objects were being initialized out of order within a single translation unit, contrary to the standard's guarantees. There is a hidden assumption there, namely that since push_back() did not cause a crash, the vector must therefore have been initialized. This turns out not to be the case: that method call on an uninitialized object is almost certainly corrupting memory somewhere, but won't necessarily cause a crash. A better way of checking whether or not the constructor is being called would be to run the code in a debugger, and set breakpoints on the lines which contain the objects' definitions, for instance std::vector<Thing*> Before in thing.cpp. This will show that initialization will occur as predicted in the standard.

The best option for avoiding the "fiasco", as described here, is "construct on first use". In the case of your example code, this would involve changing any direct use of Thing::Things, such as this line:

Thing::Things[thing->name] = thing;

To a method, say Thing::GetThings(), which initializes the object and returns a reference to it. lcs' answer provides an example of this, but beware: although it solves the static initialization problem, using a scoped static object may introduce an even more pernicious problem: crashes on program exit due to static deinitialization order. For that reason, allocating the object with the new keyword is preferred:

std::unordered_map<std::string, Thing*>& Thing::GetThings()
{
    static std::unordered_map<std::string, Thing*>* pThings =
        new std::unordered_map<std::string, Thing*>();
    return *pThings;
}

That instance will of course never be delete'd, which feels an awful lot like a memory leak. But even if it weren't a pointer, de-initialization would only occur at program shutdown. So, unless the object's destructor performs some important function like flushing a file's contents to disk, the only difference that matters is the fact that using a pointer avoids the possibility of a crash on exit.

Answer 2

As noted in the comments, static initialization order is not defined. Who knows the difference between vector and map. Maybe your compiler initializes classes with even number of characters in their name first.

If you're running c++11 or greater, static initialization of function local items is guaranteed to be thread safe. They will be initialized the first time control passes through the declaration statement.

// Header
class Thing {
public:
    static std::unordered_map<std::string, Thing*>& Things();
    static bool Register(Thing* thing);


// CPP
std::unordered_map<std::string, Thing*>& Thing::Things()
{
   static std::unordered_map<std::string, Thing*> things;
   return things;
}

This will initialize the first time you ask for the Things, and avoids all the potential randomness of static initialization.