Question

I have a global variable inside an anonymous namespace.

namespace {
std::unordered_map<std::string, std::string> m;
}


A::A() { m.insert(make_pair("1", "2")); } // crasches
void A::insert() { m.insert(make_pair("1", "2")); } // ok

If try to use the map inside the constructor I get Access violation reading location.
But if I use it after A has been initialized it works.
Is this behavior correct?

Answer 1

What is the scope of the A object whose constructor invocation is causing the crash?

There are no guarantees as to the order that static initializers are executed, so that if your A object is also a global or static (as m is), it’s quite possible that m does not exist yet in terms of being a validly constructed object, which would mean that your call to std::unordered_map::insert() would be invoked on uninitialized memory, thus leading to your crash.

A solution is to make sure that all of your A instances that depend on m are constructed explicitly by you and not statically/globally (or as the commenter added, if they are in the same TU, to order them properly), or to change the structure of A such that you can call a function on an instance later in order to do the insert. Whether or not this is a valid solution depends more on the overarching usage of A.