c++ - How to initialize object in different class? -

i'm learning c++ , have problem basics. how init object in different class?

for example have code:

class {    private:      static int num;      static string val;    public:     a(int n, string w) {                num = n;        val = w;    } }; 

i want create object in class b, have try this:

class b {    private:       obja;    public:       b(int numa, string vala){          obja = new a(numa, vala);    } }; 

different ways(same constructor):

 public:       b(a oba){         obja = oba;       } 

or

 public:       b(int numa, string vala){          obja = a(numa, vala);       } 

always i'm getting error: no default constructor exist class "a". i've read default constructor constructor without arguments, give them, why searching default?

you can following way

class b {    private:       obja;    public:       b(int numa, string vala) : obja( numa, vala ) {} };