一个计算四则表达式的模板

namespace fy_Exp{
namespace {template <class _T>
inline _T GetExpValue(_T t[], char& csym){
    char c=csym; csym=0;
    switch(c){
    case '+':return t[0] += t[1];
    case '-':return t[0] -= t[1];
    case '*':return t[0] *= t[1];
    default: return t[0] /= t[1];//case '/':
    }
}}
template <class _T, class _Tstream>
/* _Tstream: inputstream, _T: get return value
* Return nonzero if get value successfully */
int GetExpValue(_Tstream& istrin, _T& nReturn){
    _T t[3] = {0}; //雨中飞燕之作
    char csym[3] = "++";
    int nLevel = 1, nERR = 0;
    if(!(istrin>>t[1]))istrin.clear();
    for(;;){
        if(istrin>>csym[2]){
            switch(csym[2]){
            case '(':
                if(!csym[1]){nLevel=0x100; nERR=1;}else
                if(!GetExpValue(istrin, t[2]))nLevel|=0x10;
                else{nLevel=0x100; nERR=1;}
                break;
            case ')':
                {nLevel = 0x100;}break;
            case '+':case '-':case '*':case '/':
                {csym[nLevel++] = csym[2];}break;
            case ' ':case '\r':case '\n':case '\t':continue;
            default:
                {nLevel=0x100; nERR=1;}
            }
            if(nLevel==0x100)break;
            if(nLevel&0x10 || istrin>>t[2]){
                nLevel &= 0xF;
                if(nLevel==1){t[1]=t[2];csym[1]=0;continue;}
                if(csym[1]=='*'||csym[1]=='/'){
                    GetExpValue(t+1, csym[1]);
                }
                else{
                    GetExpValue(t, csym[0]);
                    t[1]=t[2];csym[0]=csym[1];csym[1]=0;
                }
                nLevel = 1;
            }
            else istrin.clear();
        }
        else{nERR = -1; break;}
    }
    if(csym[1])t[2]=0,nReturn=GetExpValue(t+1, csym[1]);
    else nReturn=GetExpValue(t, csym[0]);
    return nERR==-1?1:0;#p#分页标题#e#
}}

#include<strstream>
#include<iostream>
#include<string>
using namespace std;
int main(void)
{
    string s1;
    while(cin>>s1)
    {
        istrstream isin(s1.data());
        double d;
        if(fy_Exp::GetExpValue(isin, d))
        {
            cout<<d<<endl;
        }
        else
        {
            cout<<"ERROR"<<endl;
        }
    }
    return 0;
}