C2039字符串成员函数.pop_back(）和.back(）出错

我使用和来编程两个在整数和字符串之间交换的函数。第一个函数，字符串intToStr（int x），使用：

1) std::basic_string::push_back

它工作得很好。

但是，当第二个函数int str2Int（const string&str）使用以下成员函数时

1) std::basic_string::pop_back
2) std::basic_string::back

我得到了以下错误：

1) error C2039: 'back' : is not a member of 'std::basic_string<_Elem,_Traits,_Ax>'  
2) error C2039: 'pop_back' : is not a member of 'std::basic_string<_Elem,_Traits,_Ax>'

完整的代码如下：

#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
string intToStr(int x)
{
    bool isNegative;
    int cnt = 0;
    if(x<0)
    {
        isNegative = true;
        x = -x;
    }
    else
    {
        isNegative = false;
    }
    string s;
    while(x)
    {
        s.push_back('0'+x%10);
        x /= 10;
        cnt ++;
        if(cnt%3==0 & x!=0)
            s.push_back(',');
    }

    if(isNegative)
        s.push_back('-');
    reverse(s.begin(),s.end()); //#include <algorithm>
    return s;
}
int str2Int(const string &str)
{
    int result=0, isNegative=0;
    char temp;
    string tempStr = str;
    reverse(tempStr.begin(),tempStr.end());
     // the following code snippet doesn't work??
     // pop_back() and back() are not member function??
    while(!tempStr.empty())
    {
        temp = tempStr.back(); 
        tempStr.pop_back();
        if(temp==',')
            continue;
        else if(temp=='-')
            isNegative = 1;
        else
            result = result*10 + (temp-'0');
    }
    return isNegative? -result:result;
}