Declaring a const variable outside the class ??? Why and How

class A

        {

                 const  int    a; 

                 a=5;/////////// error

                 static const int b =6 ; running

         }

now here we can see that const int  may causes error in some compiler because

const variable specifies whether a variable is modifiable or not. But  initializing anywhere other than constructor causes default initialization followed by assignment...

so a is a const and you can't assign value to it....
Now question is that how can you define a const variable outside the class:::
                            here you can do that
                                              A():a(5)
                                                {
                           
                                                 }
whenever constructor is called value of a is initialized by 5