C互換ヘッダとstd名前空間

C++標準ライブラリに含まれるC標準ヘッダ*1と、std名前空間との関係についてメモ。

C++11以降では、下記の振る舞いが保証される：

• ヘッダcxxx: 名前空間stdにCライブラリの識別子が宣言される。グローバル名前空間にも宣言されるかもしれない。
• ヘッダxxx.h: グローバル名前空間にCライブラリの識別子が宣言される。名前空間stdにも宣言されるかもしれない。

C++17

C++17 D.5/p3-4より引用（下線部は強調）。29.9.5＝Mathematical special functions*2, 21.2.5＝byte type operations*3

3 Every other C header, each of which has a name of the form name.h, behaves as if each name placed in the standard library namespace by the corresponding cname header is placed within the global namespace scope, except for the functions described in 29.9.5, the declaration of std::byte (21.2.1), and the functions and function templates described in 21.2.5. It is unspecified whether these names are first declared or defined within namespace scope (6.3.6) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (10.3.3).
4 [Example: The header <cstdlib> assuredly provides its declarations and definitions within the namespace std. It may also provide these names within the global namespace. The header <stdlib.h> assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace std. -- end example]

C++11/14

C++11 D.5/p2-3より引用（下線部は強調）。C++14でも同様。

2 Every C header, each of which has a name of the form name.h, behaves as if each name placed in the standard library namespace by the corresponding cname header is placed within the global namespace scope. It is unspecified whether these names are first declared or defined within namespace scope (3.3.6) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (7.3.3).
3 [Example: The header <cstdlib> assuredly provides its declarations and definitions within the namespace std. It may also provide these names within the global namespace. The header <stdlib.h> assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace std. -- end example]

C++03

ヘッダcxxxでは名前空間std配下に宣言し、ヘッダxxx.hでは “名前空間std配下に宣言してからグローバル名前空間へ取り込む(using-declaration)” と強く規定されていた。C++03 D.5/p2-3より引用。

2 Every C header, each of which has a name of the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is also placed within the namespace scope of the namespace std and is followed by an explicit using-declaration (7.3.3).
3 [Example: The header <cstdlib> provides its declarations and definitions within the namespace std. The header <stdlib.h> makes these available also in the global namespace, much as in the C Standard. -- end example]

関連URL

• C++ Standard Library Defect Report List, 456. Traditional C header files are overspecified
• Why < cstdlib > is more complicated than you might think | Red Hat Developer

*1:ヘッダ <cstdlib> とヘッダ <stdlib.h> など

*2:assoc_laguerre, assoc_legendre, beta, comp_ellint_1, comp_ellint_2, comp_ellint_3, cyl_bessel_i, cyl_bessel_j, cyl_bessel_k, cyl_neumann, ellint_1, ellint_2, ellint_3, expint, hermite, laguerre, legendre, riemann_zeta, sph_bessel, sph_legendre, sph_neumann

*3:std::byte 型に対する演算子オーバーロード、to_integer<IntType>関数テンプレート