stack<Expression_Tree*> stack;

for(int i = 0; i < curr_postfix.size(); ++i) {
    if(isdigit(curr_postfix[i])) {
        cout << " kom till isdigit " << endl;
        char c = curr_postfix[i];
        int num = c;
        Expression_Tree* temp = new Integer(c);
        stack.push(temp);
        delete temp;
    }
    if(isalpha(curr_postfix[i])) {
        cout << " kom till isalpha " << endl;
        stringstream ss;
        string s;
        char str = curr_postfix[i];
        ss << str;
        ss >> s;
        Expression_Tree* temp = new Variable(s);
        stack.push(temp);
        delete temp;
    }
    if(curr_postfix[i] == '+') {
        cout << " kom till + " << endl;
        Expression_Tree* left;
        Expression_Tree* right;
        cout << " kom till slut av + " << endl;
        left = stack.top();
        stack.pop();
        right = stack.top();
        stack.pop();
        Expression_Tree* temp = new Plus(left, right);
        stack.push(temp);
        delete temp;
    }
}