468. unexpected consequences of ios_base::operator void*()

Section: 30.5.5.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-02-10

Priority: Not Prioritized

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View all issues with CD1 status.

Discussion:

The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.

    #include <cassert>
    #include <iostream>

    int main ()
    {
        assert (std::cin.tie () == std::cout);
        // calls std::cout.ios::operator void*()
    }

Proposed resolution:

Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.

Specifically, change in [lib.ios] the signature of

    operator void*() const;

to

    operator unspecified-bool-type() const;

and change [lib.iostate.flags], p1 from

    operator void*() const;

to

operator unspecified-bool-type() const;

     -1- Returns: if fail() then a value that will evaluate false in a
      boolean context; otherwise a value that will evaluate true in a
      boolean context. The value type returned shall not be
      convertible to int.

     -2- [Note: This conversion can be used in contexts where a bool
      is expected (e.g., an if condition); however, implicit
      conversions (e.g., to int) that can occur with bool are not
      allowed, eliminating some sources of user error. One possible
      implementation choice for this type is pointer-to-member.  - end
      note]

[Redmond: 5-4 straw poll in favor of doing this.]

[Lillehammer: Doug provided revised wording for "unspecified-bool-type".]