22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.17 Polymorphic function wrappers [func.wrap]

22.10.17.2 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.