class class1:
public:
...
private:
class impl; //the class that i wanna hide
std::unique_ptr<impl> impl_;
//my doxygen file:
EXTRACT_ALL = YES
EXTRACT_PRIVATE = NO
EXTRACT_PACKAGE = NO
EXTRACT_STATIC = NO
EXTRACT_LOCAL_CLASSES = YES
EXTRACT_LOCAL_METHODS = NO
HIDE_UNDOC_MEMBERS = YES
HIDE_UNDOC_CLASSES = NO