Issue 2992: system_category() and error_code::error

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2992. `system_category()` and `error_code::error_code()` should be `constexpr`

Section: 19.5.3.5 [syserr.errcat.objects], 19.5.4.2 [syserr.errcode.constructors] Status: NAD Submitter: Peter Dimov Opened: 2017-06-27 Last modified: 2017-07-12

Priority: Not Prioritized

View all other issues in [syserr.errcat.objects].

View all issues with NAD status.

Discussion:

The default constructor of error_code should be constexpr to enable constant initialization; as a practical matter, there are reports that it regularly shows up in profiles because clearing error codes is so frequent.

Suggested resolution:

add constexpr to the declaration of system_category() in [syserr.errcat.overview] and [syserr.errcat.objects];
optionally, add constexpr to the declaration of generic_category() in the same two sections;
add constexpr to the default constructor of error_code in [syserr.errcode.overview] and [syserr.errcode.constructors];
optionally, add constexpr to the error_code(int val, const error_category& cat) constructor in the same two sections;
optionally, add constexpr to error_code::assign;
optionally, add constexpr to error_code::clear;
optionally, add constexpr to error_code::value;
optionally, add constexpr to error_code::category.

There was an objection that system_category() can't be made constexpr because it needs to "immortalize" the object so that it's not destroyed at process shutdown or module unload, in order for the error_code facility to remain usable. However, the following proof of concept shows how to achieve this and still make the function constexpr:

#include <new>

template<class _Ty>
  union _Immortalizer
  { // constructs _Ty, never destroys
  constexpr _Immortalizer(): __ty()
  {
  }

  ~_Immortalizer() noexcept {}
  _Immortalizer(const _Immortalizer&) = delete;
  _Immortalizer& operator=(const _Immortalizer&) = delete;

  _Ty __ty;
};

struct error_category
{
  virtual ~error_category() = default;
};

struct system_category_impl : public error_category
{
};

[[clang::require_constant_initialization]] static const _Immortalizer<system_category_impl> _System_category;

constexpr error_category const& system_category() noexcept
{
  return _System_category.__ty;
}

struct error_code
{
  int val_;
  const error_category* cat_;

  constexpr error_code() noexcept : val_(0), cat_(&system_category()) {}

  constexpr int value() const noexcept { return val_; }
  constexpr error_category const& category() const noexcept { return *cat_; }
};

constexpr error_code s_code;

static_assert(s_code.value() == 0);
static_assert(&s_code.category() == &system_category());

[2017-07 Toronto Tuesday PM issue prioritization]

NAD; This is a feature request; needs a paper.

Proposed resolution:

This wording is relative to N4659.

Edit 19.5.3.1 [syserr.errcat.overview], class error_category synopsis, as indicated:

class error_category {
public:
  constexpr error_category() noexcept;
  virtual ~error_category();
  error_category(const error_category&) = delete;
  error_category& operator=(const error_category&) = delete;
  virtual const char* name() const noexcept = 0;
  virtual error_condition default_error_condition(int ev) const noexcept;
  virtual bool equivalent(int code, const error_condition& condition) const noexcept;
  virtual bool equivalent(const error_code& code, int condition) const noexcept;
  virtual string message(int ev) const = 0;

  bool operator==(const error_category& rhs) const noexcept;
  bool operator!=(const error_category& rhs) const noexcept;
  bool operator<(const error_category& rhs) const noexcept;
};

constexpr const error_category& generic_category() noexcept;
constexpr const error_category& system_category() noexcept;

Edit 19.5.3.5 [syserr.errcat.objects] as indicated:

constexpr const error_category& generic_category() noexcept;

[…]

constexpr const error_category& system_category() noexcept;

[…]

Edit 19.5.4.1 [syserr.errcode.overview], class error_code synopsis, as indicated:

class error_code {
public:
  // 19.5.4.2 [syserr.errcode.constructors], constructors
  constexpr error_code() noexcept;
  constexpr error_code(int val, const error_category& cat) noexcept;
  template <class ErrorCodeEnum>
  error_code(ErrorCodeEnum e) noexcept;
  
  // 19.5.4.3 [syserr.errcode.modifiers], modifiers
  constexpr void assign(int val, const error_category& cat) noexcept;
  template <class ErrorCodeEnum>
  error_code& operator=(ErrorCodeEnum e) noexcept;
  constexpr void clear() noexcept;
  
  // 19.5.4.4 [syserr.errcode.observers], observers
  constexpr int value() const noexcept;
  constexpr const error_category& category() const noexcept;
  error_condition default_error_condition() const noexcept;
  string message() const;
  explicit operator bool() const noexcept;
  
private:
  int val_; // exposition only
  const error_category* cat_; // exposition only
};

Edit 19.5.4.2 [syserr.errcode.constructors] as indicated:

constexpr error_code() noexcept;

[…]

constexpr error_code(int val, const error_category& cat) noexcept;

[…]

Edit 19.5.4.3 [syserr.errcode.modifiers] as indicated:

constexpr void assign(int val, const error_category& cat) noexcept;

[…]

constexpr void clear() noexcept;

[…]

Edit 19.5.4.4 [syserr.errcode.observers] as indicated:

constexpr int value() const noexcept;

[…]

constexpr const error_category& category() const noexcept;

[…]

2992. system_category() and error_code::error_code() should be constexpr

2992. `system_category()` and `error_code::error_code()` should be `constexpr`