The special operator
__has_cpp_attribute (operand) may be used in #if and #elif expressions in C++ code to test whether the attribute referenced by its operand is recognized by GCC.
__has_cpp_attribute (operand) is equivalent to
__has_attribute (operand) except that when operand designates a supported standard attribute it evaluates to an integer constant of the form
YYYYMM indicating the year and month when the attribute was first introduced into the C++ standard. For additional information including the dates of the introduction of current standard attributes, see SD-6: SG10 Feature Test Recommendations.