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### 2214. Clarify `basic_ios::init` call restrictions

Section: 29.5.4.2 [basic.ios.cons] Status: Open Submitter: Andrey Semashev Opened: 2012-11-09 Last modified: 2021-07-31

Priority: 4

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Discussion:

There is an ambiguity in how `std::basic_ios::init` method (29.5.4.2 [basic.ios.cons]) can be used in the derived class. The Standard only specify the state of the `basic_ios` object after the call completes. However, in `basic_ios` default constructor description (29.5.4.2 [basic.ios.cons]) there is this sentence:

> Effects: Constructs an object of class `basic_ios` (29.5.2.8 [ios.base.cons]) leaving its member objects uninitialized. The object shall be initialized by calling `basic_ios::init` before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.

This restriction hints that `basic_ios::init` should be called exactly once before the object can be used or destroyed, because `basic_ios::init` may not know whether it was called before or not (i.e. whether its members are actually uninitialized or are initialized by the previous call to `basic_ios::init`). There is no such restriction in the `basic_ios::init` preconditions so it is not clear whether it is allowed to call `basic_ios::init` multiple times or not.

This problem has already affected publicly available implementations. For example, Microsoft Visual C++ STL introduces a memory leak if `basic_ios::init` is called multiple times, while GCC 4.7 and STLPort reinitialize the `basic_ios` object correctly without memory leak or any other undesired effects. There was a discussion of this issue on Boost developers mailing list, and there is a test case that reproduces the problem. The test case is actually a bug report for my Boost.Log library, which attempts to cache `basic_ostream`-derived objects internally to avoid expensive construction and destruction. My stream objects allowed resetting the stream buffer pointers the stream is attached to, without requiring to destroy and construct the stream.

My personal view of the problem and proposed resolution follows.

While apparently the intent of `basic_ios::init` is to provide a way to initialize `basic_ios` after default construction, I see no reason to forbid it from being called multiple times to reinitialize the stream. Furthermore, it is possible to implement a conforming `basic_ios` that does not have this restriction.

The quoted above section of the Standard that describes the effects of the default constructor is misleading. The Standard does not mandate any data members of `basic_ios` or `ios_base` (29.5.2 [ios.base]), which it derives from. This means that the implementation is allowed to use non-POD data members with default constructors that initialize the members with particular default values. For example, in the case of Microsoft Visual C++ STL the leaked memory is an `std::locale` instance that is dynamically allocated during `basic_ios::init`, a raw pointer to which is stored within ios_base. It is possible to store e.g. an `unique_ptr` instead of a raw pointer as a member of `ios_base`, the smart pointer will default initialize the underlying raw pointer on default construction and automatically destroy the allocated object upon being reset or destroyed, which would eliminate the leak and allow `basic_ios::init` to be called multiple times. This leads to conclusion that the default constructor of `basic_ios` cannot leave "its member objects uninitialized" but instead performs default initialization of the member objects, which would mean the same thing in case of POD types.

However, I feel that restricting `ios_base` and `basic_ios` members to non-POD types is not acceptable. Since multiple calls to `basic_ios::init` are not forbidden by the Standard, I propose to correct the `basic_ios` default constructor description so that it is allowed to destroy `basic_ios` object without calling `basic_ios::init`. This would imply that any raw members of `basic_ios` and `ios_base` should be initialized to values suitable for destruction (essentially, this means only initializing raw pointers to NULL). The new wording could look like this:

> Effects: Constructs an object of class `basic_ios` (29.5.2.8 [ios.base.cons]) initializing its member objects to unspecified state, only suitable for `basic_ios` destruction. The object shall be initialized by calling `basic_ios::init` before its first use; otherwise the behavior is undefined.

This would remove the hint that `basic_ios::init` must be called exactly once. Also, this would remove the requirement for `basic_ios::init` to be called at all before the destruction. This is also an important issue because the derived stream constructor may throw an exception before it manages to call `basic_ios::init` (for example, if the streambuf constructor throws), and in this case the `basic_ios` destructor has undefined behavior.

To my mind, the described modification is sufficient to resolve the issue. But to emphasize the possibility to call `basic_ios::init` multiple times, a remark or a footnote for `basic_ios::init` postconditions could be added to explicitly state the semantics of calling it multiple times. The note could read as follows:

> The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.

[2013-04-20, Bristol]

Alisdair: The current wording is unclear but the proposed resolution is wrong

Solution: Clarify that `init` must be called once and only once. Move then to review.

[2021-07-29 Tim comments]

The requirement that "`init` must be called once and only once" conflicts with the disposition of LWG 135.

Proposed resolution:

This wording is relative to N3485.

1. Edit 29.5.4.2 [basic.ios.cons] as indicated:

   >     basic_ios();

   > > -2- Effects: Constructs an object of class `basic_ios` (29.5.2.8 [ios.base.cons]) leaving its member objects uninitialized initializing its member objects to unspecified state, only suitable for `basic_ios` destruction. The object shall be initialized by calling `basic_ios::init` before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.

   >     void init(basic_streambuf<charT,traits>* sb);

   > > Postconditions: The postconditions of this function are indicated in Table 128.

   > > -?- Remarks: The function can be called multiple times during the object lifetime. Each subsequent call reinitializes the object to the described in postconditions initial state.