6 Basics [basic]

6.9 Program execution [basic.exec]

6.9.3 Start and termination [basic.start]

6.9.3.3 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
 ]
A declaration D is appearance-ordered before a declaration E if
  • D appears in the same translation unit as E, or
  • the translation unit containing E has an interface dependency on the translation unit containing D,
in either case prior to E.
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 the definition of V is appearance-ordered before the definition of W, or if V has partially-ordered initialization, W does not have unordered initialization, and for every definition E of W there exists a definition D of V such that D is appearance-ordered before E, then
    • if the program does not start a thread ([intro.multithread]) other than the main thread ([basic.start.main]) or V and W have ordered initialization and they are defined in the same translation unit, the initialization of V is sequenced before the initialization of W;
    • otherwise, the initialization of V strongly happens 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.48
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, the function 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]).