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.

Table 1. C and C++ keywords
auto break case char const continue default do	double else enum extern float for goto if	int long register return short signed sizeof static	struct switch typedef union unsigned void volatile while

In C++0x, 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++0x standard adds a third usage to use this keyword to specify explicit instantiation declarations.

Standard C at the C99 level also reserves the following keywords:

Table 2. C99 keywords
_Bool _Complex	_Imaginary¹ inline restrict

Note:

The keyword _Imaginary is reserved for possible future use. For complex number functionality, use _Complex; see Complex literals for details.

The C++ language also reserves the following keywords:

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

decltype and static_assert are two keywords in the C++0x standard.

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)¹ __complex__ __const__	_Decimal32⁴ _Decimal64⁴ _Decimal128⁴ __extension__ __label__ __imag__ __inline__² pixel¹ __pixel¹ __real__	__restrict __restrict__ __signed__ __signed __static_assert⁶ __volatile__ __thread⁵ typeof³ __typeof__ vector¹ __vector¹

Note:

These keywords are recognized only in a vector declaration context, when vector support is enabled.
The __inline__ keyword uses the GNU C semantics for inline functions. For details, see Linkage of inline functions.
typeof is only recognized when -qkeyword=typeof is in effect.
These keywords are recognized only when -qdfp is enabled.
__thread is only recognized when -qtls is enabled.
__static_assert is a keyword for C language extension for compatibility with the C++0x standard.

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 of this information that describe each keyword.