把中缀转换成后缀，然后解这个方程

所以，我对这个程序有问题。有人能告诉我我哪里做错了吗?程序应该接受一个简单的中缀符号数学表达式(例如:"5 - 2 + 1")，然后将其转换为(例如:"5 2 - 1 +")然后解出来就是4。它转换得很好，但是一旦进入计算部分，它就不会在命令提示符上显示任何内容。能帮我一下吗?谢谢!

#include <iostream>
#include <stack>
#include <string>
#include <sstream>
#include <vector>
using namespace std;

int priority(string item) //prioritizing the operators
{
if(item == "(" || item == ")")
{
    return 0;
}
else if(item == "+" || item == "-")
{
    return 1;
}
else //if(item == "/" || item == "*") <-- guess didnt need to define this one
{
    return 2;
}
}
void welcome()//welcome text
{
   cout << "Welcome to this program!" << endl;
    cout << "Please enter your equation" << endl;
}
int main()
{
    welcome(); // welcome text
stack <string> myStack; // initializing the stack.
char line[256];
cin.getline( line, 256); // this and the proceeding line get the input.
string exp = line;
string item;
string postFix;
istringstream iss(exp);
iss >> item;
while(iss)
{
    if(item != "+" && item != "-" && item != "/" && item != "*" && item != "(" && item != ")") //If the char is a number
    {
        cout << item;
        postFix = postFix + item;
    }
    else if(myStack.size() == 0) // If the stack is empty
    {
        myStack.push(item);
    }
    else if( item == "+" || item == "-" || item == "/" || item == "*") //If the char is an operator
    {
        if(priority(myStack.top()) < priority(item)) // the item on the stack is greater priority than the array item
        {
            myStack.push(item);
        }
        else
        {
            while(myStack.size() > 0 && priority(myStack.top()) >= priority(item)) //while the stack contains something, and the item on 
            {           
                cout << myStack.top();
                postFix = postFix + item;
                myStack.pop();
            }
            myStack.push(item);
        }
    }
    else if(item == "(") // left peren
    {
        myStack.push(item);
    }
    else if(item == ")") // right peren
    {
        while(myStack.top() != "(")
        {
            cout << myStack.top();
            postFix = postFix + item;
            myStack.pop();
        }
        myStack.pop();
    }
    iss >> item; 
}
    while (myStack.size() > 0 ) //When nothing is left to evaluate
     {
        cout << myStack.top();
    postFix = postFix + myStack.top();
    myStack.pop();
}
   cout << endl;
    //PART 2
int x1;
int x2;
int x3;
stack<int> thirdStack;
string exp2 = postFix;
string item2;
istringstream iss2(exp2);
iss2 >> item2;
while(iss2)
    if(item2 != "+" && item2 != "-" && item2 != "/" && item2 != "*") //if its a number
    {
        int n;
        n = atoi(item2.c_str());
        thirdStack.push(n);
    }
    else if( item2 == "+" || item2 == "-" || item2 == "/" || item2 == "*") //if its an operator
    {
            x1 = thirdStack.top();
            thirdStack.pop();
            x2 = thirdStack.top();
            thirdStack.pop();
        if(item2 == "+")
        {
            x3 = x1 + x2;
            thirdStack.push(x3);
        }
        else if(item2 == "-")
        {
             x3 = x1 - x2;
             thirdStack.push(x3);
        }
        else if(item2 == "/")
        {
             x3 = x1 * x2;
             thirdStack.push(x3);
        }
        else if(item2 == "*")
        {
            x3 = x1 / x2;
            thirdStack.push(x3);
        }
    }
}
cout << "The conversion into infix notation is" + thirdStack.top() << endl;
 }