When are global variables created?

Question

I have a piece of code structured like this:

a.cpp:  
    #include "b.hpp"  
    const unsigned a =  create(1);


b.cpp:
    map<int, string> something; // global variable
    unsigned create(unsigned a){
        something.insert(make_pair(a, "somestring"));
        return a;
    }

Now, this throws out a segfault, valgrind says that map was not yet created. How does it work, how should I change it?

Answer 1

C++ does not define an order for when global variables are constructed during program startup. a could be initialized first before something gets constructed which would cause the problem above. When you start constructing global variables that depend on other global variables being initialized then you run into the classical static initialization order fiasco.

An easy way to fix your above scenario is to make something static and move it into your create function.

unsigned create(unsigned a)
{
    static map<int, string> something;
    something.insert(make_pair(a, "somestring"));
    return a;
}

That will guarantee something gets created on first call to create.