C++ Improper Function Definitions in Header Files

Explanation: Ensure that the function definitions in header files (e.g., subexpression.h, operand.h, etc.) are correctly defined.

How would I properly define the header files beyond what I have done?

subexpression.h****

class SubExpression: public Expression{

public:

SubExpression(Expression* left, Expression* right);

static Expression* parse(stringstream& in);

protected:

Expression* left;

Expression* right;

};

operand****

class Operand: public Expression{

public:

static Expression* parse(stringstream& in);

};

variable.h****

class Variable: public Operand{

public:

Variable(string name){

this->name = name;

}

int evaluate();

private:

string name;

};