Rules for Declaring Functions with noexcept

• The order of qualifiers in a function definition shall have the form.
• The noexcept operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.
• The noexcept condition must be a compile-time expression that yields a value convertible to bool.
• You cannot overload functions that have only different noexcept conditions.
• In class hierarchies, a noexcept condition is part of the specified interface. Overwriting a base class function that is noexcept in a derived class with a function that is not noexcept is an error (but not the other way around).

   class Ancestor {
   public:
      virtual void foo() noexcept {};
      virtual void foobar() noexcept {};
      virtual void bar() {};
      void baz() noexcept {};
   };
     
   class Descendant : public Ancestor {
   public:
       // OK
       void foo() noexcept override{}
     
       // ERROR, overriding cannot discard exception-specification
       //void foobar() override {};
     
       // OK, overriding can add except-specification
       void bar() noexcept override{}
     
       // OK, hiding instead of overriding
       void baz() {}
   };
     
   int main() { }
  

• Descendant class should relax exception guarantees.

   class Ancestor {
       virtual void foo1() noexcept(true) {};
       virtual void foo2() noexcept(true) {};
       virtual void foo3() noexcept(false) {};
       virtual void foo4() noexcept(false) {};
   };
     
   class Descendant : public Ancestor {
       void foo1() noexcept(true) override {};
       //void foo2() noexcept(false) override {}; // Error!
       void foo3() noexcept(true) override {};
       void foo4() noexcept(false) override {};
   };
     
   int main() { }