My static map is always empty

Question

I declared a static unordered map in a header file as follows:

static boost::unordered_map<KeyAction, sf::Key::Code> WindowKeyMap;

in that same header file, i have a function that fills the map with some values:

static void Initialize(std::string &file)
{
    WindowKeyMap[MoveLeft] = sf::Key::Code::Left;
    WindowKeyMap[MoveRight] = sf::Key::Code::Right;
    WindowKeyMap[MoveUp] = sf::Key::Code::Up;
    WindowKeyMap[MoveDown] = sf::Key::Code::Down;
    std::cout << std::endl << WindowKeyMap.size() << std::endl;
}

Later on in my program, inside a seperate class/function, i attempt to read one of the values:

std::cout << std::endl << WindowKeyMap.size() << std::endl;
auto test2 = WindowKeyMap[MoveRight];

but the map is always empty. The output to the console is always 4 from the initialize routine then 0 from the second cout. I thought static maps were persistent across the program, so I'm a little confused as to how my static map becaomes empty. Can anyone shed some light?

Thanks

Do you actually call `Initialize`... – ta.speot.is Mar 26 '11 at 09:28 — ta.speot.is, Mar 26 '11 at 09:28

score 9 · Accepted Answer · edited Mar 26 '11 at 09:31

9

When you declare your variable in the header like that each compilation unit (*.cpp) gets it's own local static copy. You have to declare it extern

extern boost::unordered_map<KeyAction, sf::Key::Code> WindowKeyMap;

and in one cpp put

boost::unordered_map<KeyAction, sf::Key::Code> WindowKeyMap;

edited Mar 26 '11 at 09:31

Björn Pollex

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answered Mar 26 '11 at 09:26

Eelke

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1

The answers to [this question](http://stackoverflow.com/questions/1358400/) explains it in more detail. – Björn Pollex Mar 26 '11 at 09:33

score 1 · Answer 2 · answered Mar 26 '11 at 09:43

Simple: just don't do it. While you can get rid of the initialization and scope as suggested by @Eelke, you will shoot yourself in the foot in the long term... Do you really want to have the hash table accessible by everybody and everywhere? Do you really accept the risk of uncontrollable access to the (apparently important) data? Do you really want to have an untestable global state all across your application? Do you really want to have all the dependencies introduced by <unordered_map> pulled into many translation units of your program? I could continue like that for a while but the point is: Wrap the logic and data into a class and provide the service via an interface. Create the instance of the interface via a factory or dependency container and manage the lifetime of that object explicitly.

Regards,

Paul

My static map is always empty

2 Answers2