20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.16 Polymorphic function wrappers [func.wrap]

20.14.16.1 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 {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.