利用find()时出现了一些未知异常

#include<iostream>
#include<fstream>
#include<vector>
using namespace std;
int main()
{
    vector<string> st;
    vector<string> st_save;
    ifstream inFile;
    inFile.open ("test.txt");
    if (!inFile.is_open())
    {
        cout<<" 打开失败了\n";
    }
    st.clear();
    st_save.clear();
    string stemp;
    while (getline(inFile, stemp))
        st.push_back(stemp);
    for (vector<string>::iterator it = st.begin(); it != st.end(); it++)
    {
        stemp = *it;
        size_t At_pace = stemp.find_first_of ('@'); //找到@的位置
        if (! (At_pace == string::npos) )
        {
            size_t first_quotation = stemp.find_last_of ('"',At_pace); //搜索@前一个引号的位置
            size_t last_quotation = stemp.find_first_of ('"',At_pace); //搜索@前一个引号的位置
            size_t st_lengtn = last_quotation - first_quotation - 1;        //取得Email Address的长度
            char st_email[st_lengtn];
            for (size_t i = 0; i < st_lengtn; ++i)
            {
                st_email[i] = stemp[first_quotation+1+i];
            }
            cout<<endl<<first_quotation<<","<<last_quotation<<","<<st_email;
            st_save.push_back(st_email);
        }
    }

    cout<<endl;
    cout<<endl;
    for (vector<string>::iterator it = st_save.begin(); it != st.end(); ++it)
    {
        cout<<*it<<endl;
    }
    cout<<"Done.\n";
    inFile.close();
    return 0;
}