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

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

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

プログラミング言語Cにおいて、ビット表現を維持したまま型変換を安全に行うマクロ。C++標準ライブラリのstd::bit_cast<To>(from)相当。

本記事の内容はStackOverflowで見つけた質問と回答に基づく。

// C99 : 右辺値に未対応
#define bit_cast(T, ...) \
  (*((T*)memcpy(&(T){0}, &(__VA_ARGS__), sizeof(T))))

// C23
#define bit_cast(T, ...) \
  ((union{typeof(T) a; typeof(__VA_ARGS__) b;}) {.b=(__VA_ARGS__)}.a)
// または
#define bit_cast(T, ...) \
  (*(typeof(T)*)memcpy(&(T){0},
    &(typeof(__VA_ARGS__)) {(__VA_ARGS__)}, sizeof(T)))

式static_assert（→id:yohhoy:20250905）を利用して型サイズ検査を組み込んだバージョン：

// C23
#define bit_cast(T, ...) \
  ((union{typeof(T) a; typeof(__VA_ARGS__) b; \
      static_assert(sizeof(T)==sizeof(__VA_ARGS__));}) \
    {.b=(__VA_ARGS__)}.a)

// C11
#define bit_cast(T, ...) \
  ((void)sizeof(struct{int dummy_member; \
     _Static_assert(sizeof(T) == sizeof(__VA_ARGS__), "");}), \
   *((T*)memcpy(&(T){0}, &(__VA_ARGS__), sizeof(T))))
// #include <assert.h>でstatic_assertと記述可能

関連URL

• cppreference bit_cast, cpprefjp bit_cast
• typeof演算子 in 標準C - yohhoyの日記
• 式static_assert in C言語 - yohhoyの日記