17 Language support library [language.support]

17.9 Exception handling [support.exception]

17.9.3 Class bad_­exception [bad.exception]

namespace std {
  class bad_exception : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}
The class bad_­exception defines the type of the object referenced by the exception_­ptr returned from a call to current_­exception ([propagation]) when the currently active exception object fails to copy.
const char* what() const noexcept override;
Returns: An implementation-defined ntbs.