C++primer--stack 처리 괄호 표현식(순서 컨테이너)

#include<iostream>
#include<string>
#include<stack>
#include<stdexcept>
#include<deque>
using namespace std;
enum obj_type {LP,RP,ADD,SUB,VAL};
struct obj
{
    obj(obj_type type, double val = 0) { t = type; v = val; }
    obj_type t;
    double v;
};
inline void skipws(string &exp, size_t &p)
{
    p = exp.find_first_not_of(" ", p);
}
inline void new_val(stack<obj>&so, double v)
{
    if (so.empty()||so.top().t==LP)
    {
        so.push(obj(VAL, v));
    }
    else if (so.top().t == ADD || so.top().t == SUB)
    {
        obj_type type = so.top().t;
        so.pop();
        if (type == ADD)
            v += so.top().v;
        else
            v = so.top().v - v;
        so.pop();
        so.push(obj(VAL, v));
    }
    else
        throw invalid_argument("     ");
}



int main(int argc, char **argv)
{
    stack<obj>so;
    string exp;
    size_t p = 0, q;
    double v;
    cout << "      " << endl;
    getline(cin, exp);
    while (p<exp.size())
    {
        skipws(exp, p);
        if (exp[p]=='(')
        {
            so.push(obj(LP));
            p++;
        }
        else if ((exp[p] == '+') || exp[p] == '-')
        {
            if (so.empty() || so.top().t != VAL)
                throw invalid_argument("     ");
            if (exp[p] == '+')
                so.push(obj(ADD));
            else
                so.push(obj(SUB));
            p++;
        }
        else if (exp[p]==')')
        {
            p++;
            if (so.empty())
                throw invalid_argument("      ");
            if (so.top().t == LP)
                throw invalid_argument("   ");
            if (so.top().t == VAL)
            {
                v = so.top().v;
                so.pop();
                if (so.empty() || so.top().t != LP)
                    throw invalid_argument("      ");
                so.pop();
                new_val(so, v);
            }
            else
                throw invalid_argument("     ");

        }
        else
        {
            v = stod(exp.substr(p), &q);
            p += q;
            new_val(so, v);
        }
    }
    if (so.size()!=1||so.top().t!=VAL)
    {
        throw invalid_argument("     ");
    }
    cout << so.top().v << endl;
    system("pause");
    return 0;
}