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

Section: 29.5.2 [ios.base], 99 [ios::failure] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-12-15 Last modified: 2017-07-30

Priority: 3

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Discussion:

29.5.2 [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.

1. Change the synopsis in 29.5.2 [ios.base] as indicated:

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

2. Change 29.5.2 [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`;

3. Change 99 [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.