Gcc/10.2.0/cpp/ 005f 005fhas 005finclude
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4.2.9 __has_include
The special operator __has_include (operand)
may be used in
‘#if
’ and ‘#elif
’ expressions to test whether the header referenced
by its operand
can be included using the ‘#include
’ directive. Using
the operator in other contexts is not valid. The operand
takes
the same form as the file in the ‘#include
’ directive (see Include Syntax) and evaluates to a nonzero value if the header can be included and
to zero otherwise. Note that that the ability to include a header doesn’t
imply that the header doesn’t contain invalid constructs or ‘#error
’
directives that would cause the preprocessor to fail.
The __has_include
operator by itself, without any operand
or
parentheses, acts as a predefined macro so that support for it can be tested
in portable code. Thus, the recommended use of the operator is as follows:
#if defined __has_include # if __has_include (<stdatomic.h>) # include <stdatomic.h> # endif #endif
The first ‘#if
’ test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used. Only when that
test succeeds is it valid to use __has_include
as a preprocessor
operator. As a result, combining the two tests into a single expression
as shown below would only be valid with a compiler that supports the operator
but not with others that don’t.
#if defined __has_include && __has_include ("header.h") /* not portable */ … #endif