C/C++ char型のビット幅定義

C23／C++23現在のプログラミング言語仕様では、char型のビット幅CHAR_BITは8bit以上の処理系定義(implementation-defined)と規定される。POSIX規格では厳密にCHAR_BIT==8と定義する。

POSIX

IEEE Std 1003.1, 2004, limits.hヘッダ仕様より一部引用。

Numerical Limits
{CHAR_BIT}
Number of bits in a type char.
　Value: 8

CHANGE HISTORY
Issue 6
The values for the limits {CHAR_BIT}, {SCHAR_MAX}, and {UCHAR_MAX} are now required to be 8, +127, and 255, respectively.

C++

C++標準ライブラリ仕様定義はC標準規格を規範的(normative)に参照している。C++23 17.3.6より一部引用。*1

#define CHAR_BIT see below

The header <climits> defines all macros the same as the C standard library header <limits.h>.
(snip)
See also: ISO C 5.2.4.2.1

C++2c(C++26)向けにCHAR_BIT==8を規定する提案文書P3477R5が検討されていたが、複数の強い反対意見(PDF)P3633R0, (PDF)P3635R0も提出され、2025年2月会合の投票では合意に至らず(not consensus)同提案は破棄された。*2

C

C23 5.2.5.3.2より一部引用（下線部は強調）。C17 5.2.4.2.1も実質的に同一内容。

The values given subsequently shall be replaced by constant expressions suitable for use in conditional expression inclusion preprocessing directives. Their implementation-defined values shall be equal or greater to those shown.

• number of bits for smallest object that is not a bit-field (byte)

CHAR_BIT  8

(snip)

関連URL

• c - Why did POSIX mandate CHAR_BIT==8? - Stack Overflow
• c - Is CHAR_BIT ever > 8? - Stack Overflow
• ［C++］WG21月次提案文書を眺める（2025年03月） - 地面を見下ろす少年の足蹴にされる私（P3477R5, P3633R0, P3635R0解説）
• cppreference: CHAR_BIT
• cpprefjp: CHAR_BIT
• C/C++ char型の符号有無 - yohhoyの日記

*1:C++23標準規格はC17（ISO/IEC 9899:2018）を引用規格(normative reference)と定める。

*2:https://github.com/cplusplus/papers/issues/2131

-fassume-nothrow-exception-dtorオプション

LLVM/Clangでは「C++例外オブジェクトのデストラクタで例外送出しないと仮定する」コンパイルオプション-fassume-nothrow-exception-dtorが提供される。こんな邪悪な例外クラスを扱うユースケースは無いだろうが、C++言語仕様上は禁止されていない。

// デストラクタで例外送出する例外オブジェクト
struct evil_exception {
  ~evil_exception() noexcept(false) { throw 42; }
};

void func()
{
  try { throw evil_exception{}; }
  catch (...) { std::puts("1st catch"); }
}

int main()
{
  try { func(); }
  catch (...) { std::puts("2nd catch"); }
}

Clangに-fassume-nothrow-exception-dtorオプション指定すると、上記evil_exceptionデストラクタはコンパイルエラーとして拒絶する。

error: cannot throw object of type 'evil_exception' with a potentially-throwing destructor

Clang 18で導入されたコンパイルオプション。

New Compiler Flags
-fassume-nothrow-exception-dtor is added to assume that the destructor of a thrown exception object will not throw. The generated code for catch handlers will be smaller. A throw expression of a type with a potentially-throwing destructor will lead to an error.

Clang 18.1.1 Release Notes — Clang 18.1.1 documentation

-fassume-nothrow-exception-dtor
Assume that an exception object' destructor will not throw, and generate less code for catch handlers. A throw expression of a type with a potentially-throwing destructor will lead to an error.

By default, Clang assumes that the exception object may have a throwing destructor. For the Itanium C++ ABI, Clang generates a landing pad to destroy local variables and call _Unwind_Resume for the code catch (...) { ... }. This option tells Clang that an exception object’s destructor will not throw and code simplification is possible.

Clang Compiler User’s Manual — Clang 18.1.1 documentation

おまけ

C++処理系毎に実行結果が異なる模様。いずれにせよ実用性は皆無。

GCC, Clang/libstdc++の実行結果：

1st catch
2nd catch

Clang/libc++の実行結果：

libc++abi: terminating due to uncaught exception of type int
Program terminated with signal: SIGSEGV

MSVC v19.50の実行結果：std::terminate呼び出し結果に相当する例外コード0xc0000409 STATUS_STACK_BUFFER_OVERRUNでプロセス異常終了する。*1

関連URL：

• ⚙ D108905 [ItaniumCXXABI] Add -fassume-nothrow-exception-dtor to assume that all exception objects' destructors are non-throwing
• [Itanium ABI] If we can make __cxa_end_catch calls nounwind conditionally · Issue #57375 · llvm/llvm-project · GitHub

*1:https://devblogs.microsoft.com/oldnewthing/20190108-00/?p=100655

冗長なtypenameキーワード

プログラミング言語C++において型(type)名を記述するとき、C++11以降では修飾名(qualified name)に限って冗長なtypenameキーワードを記述しても良い。

#include <cstddef>
using std::size_t;
struct S { using type = int; };

typename int x0;     // NG
typename size_t n0;  // NG

typename std::size_t n1;  // OK
typename ::size_t n2;     // OK
typename S::type v1;      // OK

C++03時点ではtypenameキーワードの明記はテンプレート定義内に限定されていたが、Core Working Group Issue#382により修正された経緯がある。同Issueより一部引用（下線部は強調）。

P. J. Plauger, among others, has noted that typename is hard to use, because in a given context it's either required or forbidden, and it's often hard to tell which. It would make life easier for programmers if typename could be allowed in places where it is not required, e.g., outside of templates

Notes from the April 2003 meeting:
There was unanimity on relaxing this requirement on typename. The question was how much to relax it. Everyone agreed on allowing it on all qualified names, which is an easy fix (no syntax change required). But should it be allowed other places? P. J. Plauger said he'd like to see it allowed anywhere a type name is allowed, and that it could actually be a decades-late assist for the infamous "the ice is thin here" typedef problem noted in K&R I.

Notes from October 2003 meeting:
We considered whether typename should be allowed in more places, and decided we only wanted to allow it in qualified names (for now at least).

おまけ：CWG 382で言及される “typedef problem” に関して、K&R C 1st Ed., §11.1 Lexical scopeより言及箇所を引用する。C言語では「変数名のない空の宣言*1」が許容されることと、「型名省略時の暗黙int宣言」はC99で削除されたため、現在からみると意図を読み取りづらい。

In all cases, however, if an identifier is explicitly declared at the head of a block, including the block constituting a function, any declaration of that identifier outside the block is suspended until the end of the block.

Remember also (§8.5) that identifiers associated with ordinary variables on the one hand and those associated with structure and union members and tags on the other form two disjoint classes which do not conflict. Members and tags follow the same scope rules as other identifiers. typedef names are in the same class as ordinary identifiers. They may be redeclared in inner blocks, but an explicit type must be given in the inner declaration:

typedef float distance;
...
{
    auto int distance;
    ...

The int must be present in the second declaration, or it would be taken to be a declaration with no declarators and type distance^†.

脚注† lt is agreed that the ice is thin here.

関連URL

• CWG 382. Allow typename outside of templates

*1:GCCでは警告"useless type name in empty declaration"、Clangでは警告"declaration does not declare anything"として検知される。

型パックからのインデクス指定

プログラミング言語C++において、テンプレートパラメータパック(template parameter pack)からインデクス指定で型(type)を選択する方法。C++2c(C++26)以降では直接的な記述が可能となる。

#include <tuple>

// C++11
template <std::size_t Idx, typename... Ts>
using Selector = typename std::tuple_element<Idx, std::tuple<Ts...>>::type;

// C++14
template <std::size_t Idx, typename... Ts>
using Selector = std::tuple_element_t<Idx, std::tuple<Ts...>>;

// C++2c(C++26)
template <std::size_t Idx, typename... Ts>
using Selector = Ts...[Idx];

// Selector<0, char, int, float> == char
// Selector<1, char, int, float> == int
// Selector<2, char, int, float> == float

関連URL

• (PDF) P2662R3 Pack Indexing
• cppreference: Pack indexing
• cpprefjp: パラメータパックへのインデックスアクセスを許可

unitbufマニピュレータ

C++標準ライブラリのI/Oストリームにおける知名度の低いマニピュレータ(manipulator)。*1

出力ストリームに対して出力操作毎のフラッシュ(flush)を指示する。*2

#include <iostream>

int x = 42;
std::cout << std::unitbuf;

std::cout << "x=" << x << "\n";
// operator<<呼び出し毎にflushされる

標準エラーストリームstd::cerrは初期状態でstd::ios_base::unitbufフラグがセットされている。*3

C++03 27.3.1/p3-4, 27.4, 27.4.2.1.2/Table 83, 27.4.5.1/p25-26, 27.6.2.3/p4, 27.6.2.5.1/p1より一部引用。

ostream cerr;

4 The object cerr controls output to a stream buffer associated with the object stderr, declared in <cstdio> (27.8.2).
5 After the object cerr is initialized, cerr.flags() & unitbuf is nonzero. Its state is otherwise the same as required for basic_ios<char>::init (27.4.4.1).

#include <iosfwd>
namespace std {
  // 27.4.5, manipulators:
  // (snip)
  ios_base& unitbuf  (ios_base& str);
  ios_base& nounitbuf(ios_base& str);
  // (snip)
}

Table 83 - fmtflags effects

Element	Effect(s) if set
unitbuf	flushes output after each output operation;

ios_base& unitbuf(ios_base& str);

25 Effects: Calls str.setf(ios_base::unitbuf).
26 Returns: str.

˜sentry();

4 If ((os.flags() & ios_base::unitbuf) && !uncaught_exception()) is true, calls os.flush().

1 Each formatted output function begins execution by constructing an object of class sentry. (snip)

関連URL

• cppreference: std::unitbuf, std::nounitbuf
• cpprefjp: std::unitbuf, std::nounitbuf
• boolalphaマニピュレータ - yohhoyの日記
• endl, ends, flushの正体 - yohhoyの日記

*1:当社比：https://x.com/yohhoy/status/2011376066376696061

*2:ライブラリ仕様上は basic_ostream::sentry クラスのデストラクタでフラッシュ操作が規定される。出力関数の開始時に同クラスオブジェクトを作成し、終了時にデストラクタが呼び出される。

*3:ワイド文字標準エラーストリーム std::wcerr も同様に、初期状態で unitbuf フラグが設定される。

"obsolescent feature" in 標準C

プログラミング言語C標準規格に登場する「廃止予定の機能(obsolescent feature)」の意味を正確に規定する提案が提出されている。

Motivation
Mixing the terms "obsolete", "obsolescent", "deprecated", and "removed", despite distinctions in their meanings, sends confusing and inconsistent message to the outside world, leading to misunderstandings even among the members of the Committee.

This paper aims to clarify the terminology by defining term "obsolete" and the categories of such state.

The Standard itself ought to use the precise categories for individual features. The word "obsolete" is an umbrella term, already seen as existing practice in community (see: cppreference.com, Wikipedia), which would also be useful for papers that put to a discussion the degree to which a feature can be obsoleted (e.g. N3353). As an added bonus, defining it highlights existence of a distinction between "obsolete" and "obsolescent", which may not be clear to all readers otherwise.

N3818 Clarify terminology for obsolete features, v2

C23現在はdeprecated属性（→id:yohhoy:20200505）で間接的に定義されているのみ。C23 6.7.13.5/p3より引用。

Semantics
The deprecated attribute can be used to mark names and entities whose use is still allowed, but is discouraged for some reason.¹⁷⁴⁾
脚注174) In particular, deprecated is appropriate for names and entities that are obsolescent, insecure, unsafe, or otherwise unfit for purpose.

C++

C++標準規格ではAnnex Dにて用語 "deprecated" が定義される。C++03/p2より引用。

These are deprecated features, where deprecated is defined as: Normative for the current edition of the Standard, but not guaranteed to be part of the Standard in future revisions.

関連URL

• 8進数リテラルプレフィクス in 標準C - yohhoyの日記 （N3353の内容）
• realloc(ptr, 0)は廃止予定 - yohhoyの日記
• C2x標準の属性(attribute) - yohhoyの日記
• gets関数の最期 - yohhoyの日記

fflush(NULL);

C標準ライブラリ関数fflushにヌルポインタを指定すると、プログラムが現在開いている全てのストリームをフラッシュする。

fflush(NULL);  // OK

C99 7.19.5.2/p1-3より引用（下線部は強調）。

#include <stdio.h>
int fflush(FILE *stream);

2 If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes anyunwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.
3 If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above.

(PDF) C99 Rationale, 7.19.5.2より引用。

The fflush function ensures that output has been forced out of internal I/O buffers for a specified stream. Occasionally, however, it is necessary to ensure that all output is forced out, and the programmer may not conveniently be able to specify all the currently open streams, perhaps because some streams are manipulated within library packages. To provide an implementation-independent method of flushing all output buffers, the Standard specifies that this is the result of calling fflush with a NULL argument.

関連URL

• _Exit/_exitはストリームをフラッシュしない - yohhoyの日記