一个奇妙的段错误

#include <iostream>
#include <unordered_map>
#include <stack>
#include <algorithm>

using namespace std;


unordered_map<char, int> pr {{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};

stack<int> num;
stack<char> op;

void eval()
{
    int a, b, x; 
    char c;
    b = num.top(); num.pop();
    a = num.top(); num.pop();
    c = op.top(); op.pop();
    if(c == '+') x = a + b;
    else if(c == '-') x = a- b;
    else if(c == '*') x = a * b;
    else x = a / b;
    num.push(x);
}

int main()
{
    string str;
    cin >> str;


    for (int i = 0; i < str.size(); i ++ )
    {
        if (isdigit(str[i]))
        {
            int j = i, x = 0;
            while (isdigit(str[j]) && j < str.size())
                x = x * 10 + str[j ++ ] - '0';
            num.push(x);
            i = j - 1;
        }
        else if (str[i] == '(') op.push(str[i]);
        else if (str[i] == ')')
        {
            while (op.top() != '(') eval();
            op.pop();
        }
        else
        {
            //错误类型：当栈为空时，调用top()会导致段错误
            //while (pr[op.top()] >= pr[str[i]] && op.size()) eval();
            while (op.size() && pr[op.top()] >= pr[str[i]]) eval();
            op.push(str[i]);
        }
    }
    while(op.size()) eval();
    cout << num.top() << endl;

    return 0;
}