369. io stream objects and static ctors

Section: 30.4 [iostream.objects] Status: CD1 Submitter: Ruslan Abdikeev Opened: 2002-07-08 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?

Surpisingly enough, Standard does NOT require that.

27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.

However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.

Details:

Core text refers to some magic object ios_base::Init, which will be discussed below:

"The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])

The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.

However, the second non-normative footnote makes an explicit and unsupported claim:

"Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])

The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.

However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.

Proposed resolution:

Add to 30.4 [iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:

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, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.

[Lillehammer: Matt provided wording.]

[Mont Tremblant: Matt provided revised wording.]

Rationale:

The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.

The new proposed resolution gives users guidance on what they need to do to ensure that stream objects are constructed during startup.