Solving order of initialization:
First off, this is just a temporary work-around because you have global variables that you are trying to get rid of but just have not had time yet (you are going to get rid of them eventually aren't you? :-)
class A
{
public:
// Get the global instance abc
static A& getInstance_abc() // return a reference
{
static A instance_abc;
return instance_abc;
}
};
This will guarantee that it is initialised on first use and destroyed when the application terminates.
Multi-Threaded Problem:
C++11 does guarantee that this is thread-safe:
§6.7 [stmt.dcl] p4
If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.
However, C++03 does not officially guarantee that the construction of static function objects is thread safe. So technically the getInstance_XXX() method must be guarded with a critical section. On the bright side, gcc has an explicit patch as part of the compiler that guarantees that each static function object will only be initialized once even in the presence of threads.
Please note: Do not use the double checked locking pattern to try and avoid the cost of the locking. This will not work in C++03.
Creation Problems:
On creation, there are no problems because we guarantee that it is created before it can be used.
Destruction Problems:
There is a potential problem of accessing the object after it has been destroyed. This only happens if you access the object from the destructor of another global variable (by global, I am referring to any non-local static variable).
The solution is to make sure that you force the order of destruction.
Remember the order of destruction is the exact inverse of the order of construction. So if you access the object in your destructor, you must guarantee that the object has not been destroyed. To do this, you must just guarantee that the object is fully constructed before the calling object is constructed.
class B
{
public:
static B& getInstance_Bglob;
{
static B instance_Bglob;
return instance_Bglob;;
}
~B()
{
A::getInstance_abc().doSomthing();
// The object abc is accessed from the destructor.
// Potential problem.
// You must guarantee that abc is destroyed after this object.
// To guarantee this you must make sure it is constructed first.
// To do this just access the object from the constructor.
}
B()
{
A::getInstance_abc();
// abc is now fully constructed.
// This means it was constructed before this object.
// This means it will be destroyed after this object.
// This means it is safe to use from the destructor.
}
};
