What are _if_exists and __if_not_exists The __if_exists and __if_not_exists keywords in the Active Template Library (ATL) allow a user to test at compile time whether an identifier exists.  If the identifier exists, the associated statement block is executed.  __if_exists and __if_not_exists can be applied to the names of variables,


원문출처 : https://devblogs.microsoft.com/cppblog/an-alternative-to-__if_exists-in-atl