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Section: 99 [ios::Init] Status: C++11 Submitter: James Kanze Opened: 2009-05-14 Last modified: 2018-05-20
Priority: Not Prioritized
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As currently formulated, the standard doesn't require that there is ever a flush of cout, etc. (This implies, for example, that the classical hello, world program may have no output.) In the current draft (N2798), there is a requirement that the objects be constructed before main, and before the dynamic initialization of any non-local objects defined after the inclusion of <iostream> in the same translation unit. The only requirement that I can find concerning flushing, however, is in 99 [ios::Init], where the destructor of the last std::ios_base::Init object flushes. But there is, as far as I can see, no guarantee that such an object ever exists.
Also, the wording in [iostreams.objects] says that:
The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution.
In 99 [ios::Init], however, as an effect of the constructor, it says that
If init_cnt is zero, the function stores the value one in init_cnt, then constructs and initializes the objects cin, cout, cerr, clog wcin, wcout, wcerr, and wclog"
which seems to forbid earlier construction.
(Note that with these changes, the exposition only "static int init_cnt" in ios_base::Init can be dropped.)
Of course, a determined programmer can still inhibit the flush with things like:
new std::ios_base::Init ; // never deleted
or (in a function):
std::ios_base::Init ensureConstruction ; // ... exit( EXIT_SUCCESS ) ;
Perhaps some words somewhere to the effect that all std::ios_base::Init objects should have static lifetime would be in order.
[ 2009 Santa Cruz: ]
Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove init_cnt from Init.
Change 29.4 [iostream.objects]/2:
-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class ios_base::Init is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293
If a translation unit includes <iostream> or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit.
Change 99 [ios::Init]/3:
-3- Effects: Constructs an object of class Init.
If init_cnt is zero, the function stores the value one in init_cnt, then constructs and initializes the objects cin, cout, cerr, clog (27.4.1), wcin, wcout, wcerr, and wclog (27.4.2). In any case, the function then adds one to the value stored in init_cnt.
Change 99 [ios::Init]/4:
-4- Effects: Destroys an object of class Init.
The function subtracts one from the value stored in init_cnt and, if the resulting stored value is one,calls cout.flush(), cerr.flush(), clog.flush(), wcout.flush(), wcerr.flush(), wclog.flush().