20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.16 Polymorphic function wrappers [func.wrap] Class bad_­function_­call [func.wrap.badcall]

An exception of type bad_­function_­call is thrown by function​::​operator() ([func.wrap.func.inv]) when the function wrapper object has no target.
namespace std {
  class bad_function_call : public exception {
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
Returns: An implementation-defined ntbs.