Issue 2462: std::ios_base::failure is overspecified

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2462. `std::ios_base::failure` is overspecified

Section: 31.5.2 [ios.base], 31.5.2.2.1 [ios.failure] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-12-15 Last modified: 2023-02-07

Priority: 3

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Discussion:

31.5.2 [ios.base] defines ios_base::failure as a nested class:

namespace std {
  class ios_base {
  public:
    class failure;
    […]
  };
  […]
}

This means it is valid to use an elaborated-type-specifier to refer to ios_base::failure:

using F = class std::ios_base::failure;
throw F();

Therefore implementations are not permitted to define ios_base::failure as a typedef e.g.

 class ios_base {
 public:
#if __cplusplus < 201103L
   class failure_cxx03 : public exception {...};
   typedef failure_cxx03 failure;
#else
   class failure_cxx11 : public system_error {...};
   typedef failure_cxx11 failure;
#endif
   […]
 };

This constrains implementations, making it harder to manage the ABI change to ios_base::failure between C++03 and C++11.

[2015-05-06 Lenexa: Move to Ready]

JW: the issue is that users are currently allowed to write "class failure" with an elaborated-type-specifier and it must be well-formed, I want the freedom to make that type a typedef, so they can't necessarily use an elaborated-type-specifier (which there is no good reason to use anyway)

JW: ideally I'd like this everywhere for all nested classes, but that's a paper not an issue, I only need this type fixed right now.

RD: is a synonym the same as an alias?

JW: dcl.typedef says a typedef introduces a synonym for another type, so I think this is the right way to say this

JW: I already shipped this last month

PJP: we're going to have to break ABIs again some time, we need all the wiggle room we can get to make that easier. This helps.

MC: do we want this at all? Ready?

9 in favor, none opose or abstaining

Proposed resolution:

This wording is relative to N4296.

Change the synopsis in 31.5.2 [ios.base] as indicated:

namespace std {
  class ios_base {
  public:
    class failure; // see below
    […]
  };
  […]
};

Change 31.5.2 [ios.base] paragraph 1:
ios_base defines several member types:
- a ~~class failure~~type failure, defined as either a class derived from system_error or a synonym for a class derived from system_error;
Change [ios::failure] paragraph 1:

-1- An implementation is permitted to define ios_base::failure as a synonym for a class with equivalent functionality to class ios_base::failure shown in this subclause. [Note: When ios_base::failure is a synonym for another type it shall provide a nested type failure, to emulate the injected class name. — end note] The class failure defines the base class for the types of all objects thrown as exceptions, by functions in the iostreams library, to report errors detected during stream buffer operations.

2462. std::ios_base::failure is overspecified

2462. `std::ios_base::failure` is overspecified