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Re: [patch] libstdc++/56158 Extend valid values of iostream bitmask types


On 11/11/2015 02:48 AM, Jonathan Wakely wrote:
As described in the PR, we have operator~ overloads defined for
enumeration types which produce values outside the range of valid
values for the type. In C++11 that can be trivially solved by giving
the enumeration types a fixed underlying type, but this code needs to
be valid in C++03 too.

This patch defines new min/max enumerators as INT_MIN/INT_MAX so that
every int value is also a valid value for the bitmask type.

Does anyone see any problems with this solution, or better solutions?

Just a minor nit that the C-style cast in the below triggers
a -Wold-style-cast warning in Clang, in case libstdc++ tries
to be Clang-warning free. Since the type of __INT_MAX__ is
int it shouldn't be necessary.

+      _S_ios_fmtflags_min = ~(int)__INT_MAX__


Any suggestions for how to test this, given that GCC's ubsan doesn't
check for this, and we can't run the testsuite with ubsan anyway?

Use a case/switch statement with -Werror=switch-enum to make sure
all the cases are handled and none is duplicated or outside of the
valid values of the enumeration:

  void foo (ios::iostate s) {
      switch (s) {
      case badbit:
      case eofbit:
      case failbit:
      case goodbit:
      case __INT_MAX__:
      case ~__INT_MAX__: ;
    }
  }

Martin


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