c++ 简易计算器 back() called on empty deque 问题错误

Question

#include<iostream>
#include<string>
#include<stack>
#include<cmath>
using namespace std;

class jisuanqi
{
private:
    string a;
    stack <int>s;
    stack<char>f;
public:
    jisuanqi()
    {
        a = "3+2*(5+1)=";
    }
    void jisuan();
    int youxianji(char a);
    int suan(char a,int xia,int shang);
};
int jisuanqi::suan(char a,int xia,int shang)
{
    switch (a)
    {
    case '+':return xia + shang;
    case '-':return xia - shang;
    case '/':return xia / shang;
    case '*': return xia * shang;
    }
}
int jisuanqi::youxianji(char a)
{
    switch (a)
    {
    case ')': return 0;
        break;
    case '+':return 1;
        break;
    case '-':return 1;
        break;
    case '*':return 2;
        break;
    case '/':return 2;
        break;
    case '(':return 3;
        break;
    default:
        return -1000;
        cout << "符号输入错误";
    }
}
void jisuanqi::jisuan()
{
    char* b = &a[0];

    while (*b != '=')
    {
        int num = 0;
        int k = 0;
        if (*b >= '0' && *b <='9')
        {
            char* c = (b + 1);
            while (*c <= '9' && *c>='0')
            {
                num++;
                c = (c + 1);
            }
            while (num != 0)
            {
                k = k + (int(*b)-48) * pow(10, num);
                num--;
                b++;
            }
            s.push(k + int(*b)-48);
            b++;

        }
        else
        {
            if(f.top()=='(')
            {
                f.push(*b);
                b++;
                continue;
            }
            if(f.top()=='('&&*b==')')
            {
                f.pop();
                b++;
                continue;
            }
            if (f.empty())
            {
                f.push(*b);
                b++;
                continue;
            }
            else if (!f.empty()&&youxianji(f.top() )>= youxianji(*b))
            {
                int t = s.top();
                s.pop();
                int r = s.top();
                s.pop();
                s.push(suan(f.top(), r, t));
                f.pop();
            }
            else if (!f.empty()&&youxianji(f.top()) < youxianji(*b))
            {
                f.push(*b);
            }
            b++;
        }
    
    }
    while(!f.empty()&&!s.empty())
    {
        int q = s.top();
        s.pop();
        int z = s.top();
        s.pop();
        s.push(suan(f.top(), z, q));
        f.pop();
    }
    cout << s.top();
    return;
}

int main()
{
    jisuanqi x;
    x.jisuan();
    system("pause");
    return 0;
}

简易计算器，请问，错误大概是什么类型的？如何解决？

Answer 1

可能是因为一开始栈为空，此时调用top函数会报错
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