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 {
    // [func.wrap.badcall.const], constructor
    bad_function_call() noexcept;
} Constructor [func.wrap.badcall.const]

bad_function_call() noexcept;
Effects: Constructs a bad_­function_­call object.
Ensures: what() returns an implementation-defined ntbs.