How to make Nested class, which use default constructor parameters?

[osfreak]

class Outer
{
public:
	int outera;

	class Inner{

	public:
		static int a,b;
		int c;
		void Displaystatvalue();
		Inner(int parm = 0):c(parm)
		{
		}
		
	}in1;	     //}in1(15);//Not allowed why
        Inner in2;   //Inner in2(10);//not allowed,why?

};

[weaknessforcats]

in1 and in2 are member variables. in(15) and in2(10) are constructor calls. That is, actual code that executes and allocates memory. in(15)and in2(10) are , therefore definitions (objects created at run time), and you cannot have definitions inside a declaration because there's no way to make those calls at compile time.

[Banfa]

If you want to initialise the members of a class then you do it in the class initialisation list.

The initialisation list is something that can only be attached to a constructor and it specifies how to initialise class members as well as base classes.

In your case something like

Outer::Outer() :
  in2(10)
{
}

in much the same way you initialise the C member of Inner