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This is an older version of Boost and was released in 2018. The current version is 1.89.0.
boost::type_erasure::placeholder_of
// In header: <boost/type_erasure/placeholder_of.hpp> template<typename T> struct placeholder_of { // types typedef unspecified type; };
A metafunction returning the (const/reference qualified) placeholder corresponding to an any. It will also work for all bases of any, so it can be applied to the Base parameter of concept_interface.