6 Basic concepts [basic]

6.8 Program execution [basic.exec]

6.8.3 Start and termination [basic.start]

6.8.3.3 Dynamic initialization of non-local variables [basic.start.dynamic]

Dynamic initialization of a non-local variable with static storage duration is unordered if the variable is an implicitly or explicitly instantiated specialization, is partially-ordered if the variable is an inline variable that is not an implicitly or explicitly instantiated specialization, and otherwise is ordered.
[Note
:
An explicitly specialized non-inline static data member or variable template specialization has ordered initialization.
end note
]
Dynamic initialization of non-local variables V and W with static storage duration are ordered as follows:
  • If V and W have ordered initialization and V is defined before W within a single translation unit, the initialization of V is sequenced before the initialization of W.
  • If V has partially-ordered initialization, W does not have unordered initialization, and V is defined before W in every translation unit in which W is defined, then
    • if the program starts a thread ([intro.multithread]) other than the main thread ([basic.start.main]), the initialization of V strongly happens before the initialization of W;
    • otherwise, the initialization of V is sequenced before the initialization of W.
  • Otherwise, if the program starts a thread other than the main thread before either V or W is initialized, it is unspecified in which threads the initializations of V and W occur; the initializations are unsequenced if they occur in the same thread.
  • Otherwise, the initializations of V and W are indeterminately sequenced.
[Note
:
This definition permits initialization of a sequence of ordered variables concurrently with another sequence.
end note
]
A non-initialization odr-use is an odr-use ([basic.def.odr]) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.
It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred.
If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized.55
It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.
[Note
:
Such points should be chosen in a way that allows the programmer to avoid deadlocks.
end note
]
[Example
:
// - File 1 -
#include "a.h"
#include "b.h"
B b;
A::A(){
  b.Use();
}

// - File 2 -
#include "a.h"
A a;

// - File 3 -
#include "a.h"
#include "b.h"
extern A a;
extern B b;

int main() {
  a.Use();
  b.Use();
}
It is implementation-defined whether either a or b is initialized before main is entered or whether the initializations are delayed until a is first odr-used in main.
In particular, if a is initialized before main is entered, it is not guaranteed that b will be initialized before it is odr-used by the initialization of a, that is, before A​::​A is called.
If, however, a is initialized at some point after the first statement of main, b will be initialized prior to its use in A​::​A.
end example
]
It is implementation-defined whether the dynamic initialization of a non-local inline variable with static storage duration is sequenced before the first statement of main or is deferred.
If it is deferred, it strongly happens before any non-initialization odr-use of that variable.
It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.
It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred.
If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized.
It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.
If the initialization of a non-local variable with static or thread storage duration exits via an exception, std​::​terminate is called ([except.terminate]).
A non-local variable with static storage duration having initialization with side effects is initialized in this case, even if it is not itself odr-used ([basic.def.odr], [basic.stc.static]).