Feature-testing preprocessor predicates for C++17 (original) (raw)
defined-macro-expression:
defined
identifier
defined (
identifier )
h-preprocessing-token:
any preprocessing-token other than >
h-pp-tokens:
h-preprocessing-token
h-pp-tokens h-preprocessing-token
has-include-expression:
__has_include ( <
h-char-sequence> )
__has_include ( "
q-char-sequence" )
__has_include (
string-literal )
__has_include ( <
h-pp-tokens > )
The expression that controls conditional inclusion shall be an integral constant expression except that identifiers (including those lexically identical to keywords) are interpreted as described below146 and it may contain zero or more defined-macro-expressions and/or has-include-expressions as unary operator expressions. of the form
defined
identifier
or
defined (
identifier )
which evaluate A defined-macro-expression evaluates to 1
if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define
preprocessing directive without an intervening #undef
directive with the same subject identifier),0
if it is not.
The third and fourth forms of has-include-expression are considered only if neither of the first or second forms matches, in which case the preprocessing tokens are processed just as in normal text.
The header or source file identified by the parenthesized preprocessing token sequence in each contained has-include-expression is searched for as if that preprocessing token sequence were the pp-tokens in a #include
directive, except that no further macro expansion is performed. If such a directive would not satisfy the syntactic requirements of a#include
directive, the program is ill-formed. The has-include-expression evaluates to 1
if the search for the source file succeeds, and to 0
if the search fails.
The #ifdef
and #ifndef
directives, and the defined
conditional inclusion operator, shall treat __has_include
as if it were the name of a defined macro. The identifier __has_include
shall not appear in any context not mentioned in this section.