Keywords

Keywords are identifiers reserved by the language for special use. Although you can use them for preprocessor macro names, it is considered poor programming style. Only the exact spelling of keywords is reserved. For example, auto is reserved but AUTO is not.

Keywords for the C and C++ languages

Table 1. C and C++ keywords
auto¹ break case char const continue default do	double else enum extern² float for goto if	int long register return short signed sizeof static	struct switch typedef union unsigned void volatile while
Notes: In C++11, the keyword `auto` is no longer used as a storage class specifier. Instead, it is used as a type specifier, which can deduce the type of an `auto` variable from the type of its initializer expression. The keyword `extern` was previously used as a storage specifier or as part of a linkage specification. The C++11 standard adds a third usage to use this keyword to specify explicit instantiation declarations.

Keywords for the C language only

Standard C at the C99 and C11 levels also reserves the following keywords:

Table 2. C99 and C11 keywords
_Bool _Complex _Generic¹ _Imaginary² _Noreturn¹	_Static_assert¹ inline restrict
Notes: This keyword is introduced due to the C11 language level. The keyword `_Imaginary` is reserved for possible future use. For complex number functionality, use `_Complex`; see Complex literals for details.

Keywords for the C++ language only

The C++ language also reserves the following keywords:

Table 3. C++ keywords
asm bool catch class const_cast constexpr delete	dynamic_cast decltype explicit export false friend inline mutable namespace	new nullptr operator private protected public reinterpret_cast static_assert static_cast template	this throw true try typeid typename using virtual wchar_t

Keywords for language extensions (IBM extension)

In addition to standard language keywords, XL C/C++ reserves the following keywords for use in language extensions:

Table 4. Keywords for C and C++ language extensions
__alignof __alignof__ __asm (C only) __asm__ (C only) _attribute__ __attribute bool (C only)⁸ __complex__ __const__ _Decimal32³ _Decimal64³ _Decimal128³ __decltype⁷	__extension__ __label__ __imag__ __inline__¹ _Noreturn⁶ pixel⁸ __pixel⁸ __real__ __restrict __restrict__ __signed__ __signed	__static_assert⁵ __volatile __volatile__ __thread⁴ typeof² __typeof__ vector⁸ __vector⁸
Notes: The `__inline__` keyword uses the GNU C semantics for inline functions. For details, see Linkage of inline functions. `typeof` is recognized only when -qkeyword=typeof is in effect. These keywords are recognized only when -qdfp is enabled. `__thread` is recognized only when -qtls is enabled. `__static_assert` is a keyword for C language extension for compatibility with the C++11 standard. `_Noreturn` is a keyword for C++ language extension for compatibility with the C11 standard. `__decltype` is a keyword for C++ language extension. It enables you to use the C++11 decltype functionality in all language levels. These keywords are recognized only in a vector declaration context, when vector support is enabled.

XL C++ reserves the following keywords as language extensions for compatibility with C99.

Table 5. Keywords for C++ language extensions related to C99
_Complex _Imaginary¹ _Pragma restrict
Note: The keyword `_Imaginary` is reserved for possible future use. For complex number functionality, use `_Complex`; see Complex literals for details.

More detailed information regarding the compilation contexts in which extension keywords are valid is provided in the sections that describe each keyword.