C / C++ / MFC

//Method 1:  Constructor in Struct
class OuterClass
{
public:
	OuterClass() {}
	~OuterClass() {}

private:
	struct InnerClass
	{
		InnerClass() { _data = 0; }
		int _data;
	};

	InnerClass _MyData;
};

// Method 2:  Initialize Method
class OuterClass
{
public:
	OuterClass() {}
	~OuterClass() {}

	void Initialize() { _MyData._data = 0; }
private:
	struct InnerClass
	{
		int _data;
	};

	InnerClass _MyData;
};

// example structure
typedef struct TESTSTRUCT {
 double a;
}
// base class 
class testA 
{
public:
 TESTSTRUCT st;
...
};
// another class with members derived from base class and implied as private members
class testB: private testA 
{
 public:
  void MyFunc(double val) {
   st.a = val;
  };
...
};

int main (...) {
 testB c;
 c.st.a = 1.1; // C2248 error, you're trying to access private member
 Myfunc(1.1);  // OK :) private member is accessed by publicly available function, not directly.
}