c++实现四则运算

#include<iostream>
using namespace std;
double expr(char* str);
const int MAX=80;              //待处理的最大字符串长度
void eatspace(char* str){       //去掉空格 
    int i=0;
    int j=0;
    while(str[i]!='\0')
        if(str[i]==' '){                
            i++;
        }
        else{
            str[j]=str[i];
            i++; 
            j++;
        }
    str[j]='\0';

}
char* extract(char* str,int& index){        //返回括号之间的expr
    int count=1;
    char* substr=nullptr;
    int substr_begin=index;
    int substr_end=index;
    char buffer[MAX];

    while(str[index]!='\0'){
        buffer[index-substr_begin]=str[index];

        switch(str[index++]){
        case '(': count++; break;
        case ')': 
            count--; 
            substr_end=index-1;
            int length=substr_end-substr_begin;
            if(0==count){
                 buffer[length]='\0';
                 substr=new char[length+1];
                 if(!substr){
                     cout<<"Memory allocation failed,programme terminated!!";
                     exit(1);
                 }
                 else{
                     strcpy_s(substr,length+1,buffer);
                     return substr;
                 }
            }
           
            break;
        }
    }
    if(count>0){
        cout<<endl<<"括号不匹配！！"<<endl;
        exit(1);
    }

    
            
}
double number(char* str,int& index){       //返回number
    double value=0.0;
    double factor=1.0;
    if(str[index]=='('){
        char* psubstr=nullptr;
        psubstr=extract(str,++index);
        value=expr(psubstr);
        delete[] psubstr;
        psubstr=nullptr;
        return value;
    }

    if(!isdigit(str[index])){
        cout<<endl<<"There is a number error!!!!!"<<endl;
        exit(1);
    }
    while(isdigit(str[index])){
        value=10*value+(str[index++]-'0');
    }
    if(str[index]!='.'){
        return value;
    }
    else{
        while(isdigit(str[++index])){
            factor*=0.1;
            value+=(str[index]-'0')*factor;
        }
    }

    return value;

}
double term(char* str,int& index){        //返回term
    double value=0.0;
    value=number(str,index);

    while(true){
        switch(str[index++]){
        case '*': value*=number(str,index);  break;
        case '/': value-=number(str,index);   break;
        default: index--; return value;
        }
    }

}
double expr(char* str){          //返回expr
    double value=0.0;    
    int index=0;

    value=term(str,index);
    while(true){
        switch(str[index++]){
        case '+': value+=term(str,index); break;
        case '-': value-=term(str,index); break;
        case '\0': return value;
        default:  cout<<endl<<"There is an expr error!!!!!!"<<endl; exit(1);
        }
    }
    return value;

}
int main(void ){
    
    char buffer[MAX];
    cin.getline(buffer,MAX);

    eatspace(buffer);
    cout<<"After eaptspace:"<<endl;
    for(int i=0;i<MAX;i++)
        if(buffer[i]!='\0')
            cout<<buffer[i];
        else
            break;

    cout<<"\t="<<expr(buffer)<<endl;
    return 0;
}