c++ 编译错误 - 体系结构x86_64的未定义符号

可能的重复项：
架构 i386 的未定义符号：

using namespace std;
#include <iostream>
#include <fstream>
#include <string>
#include "CorpusExp.h"
int main(int argc, char* argv[]){
ifstream infile("../DATA.txt");
string train_dir;   // The training data directory
infile>>train_dir;
train_dir=train_dir.substr(6,train_dir.length());
if (argc>1){
    if (strcmp(argv[1],"-s")){ // enter into CorpusExploration mode
        CorpusExp ce(train_dir,"all");   //<<=======LINE X!!!!!!!!
        //ce.calculate();
        if (argc>=3 && strcmp(argv[2],"-u")){ // check user stats
            cout<<"shit";
        }
        else if (argc>=3 && strcmp(argv[2],"-m")){ // check movie stats
        }else{ // check the all (default) stats
        }
    }else if(strcmp(argv[1],"-m")) {// enter into Recommendation mode
    }
}
return 0;
}

如果我包含 LINE X，主.cpp将无法编译，错误消息是：

      Undefined symbols for architecture x86_64:
   "CorpusExp::CorpusExp(std::basic_string<char, std::char_traits<char>,     std::allocator<char> > const&, std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
make: *** [recommend] Error 1

.cpp和CorpusExp.h：

/* CorpusExporater 头文件 用于统计目的

*/

using namespace std;
#include <iostream>
#include <fstream>
#include <string>
#include <dirent.h>  // the Linux library to read the directories
class CorpusExp{
public:
    CorpusExp(const string&,const string&); // (dir_name, mode)
    void calculate();
    void display(ostream &out, const int&);    // print out the id's stat information
private:
    string _dir_name;
    int _movie_count;
    int _user_count;
    int _movie_rated_count[5]; // times of movie rated as 1,2,3,4,5
    float _movie_avg;   // total moview average rates
    string _mode; // the mode of current task
    int _id; // the id of current task
};
/*CorpusExporater cpp file
   Used for statistical purpose
*/
#include "CorpusExp.h"
CorpusExp::CorpusExp(const string &dir_name, const string &mode):
    _dir_name(dir_name),
    _mode(mode),
    _movie_count(0),
    _user_count(0)
{

}
void CorpusExp::calculate(){
    DIR *dpdf;
    struct dirent *epdf;
    dpdf = opendir(strdup(_dir_name.c_str()));
    if (dpdf!=NULL){
        while(epdf = readdir(dpdf)){
            cout<<epdf->d_name<<endl;
        }
    }
}
void CorpusExp::display(ostream &out, const int &id){
}

我对c ++几乎不陌生，所以我很困惑，不知道是哪个部分导致了这个问题。如果我删除 LINE X，一切都会好起来的，但是如果我在主函数中声明 CorpusExp 类，那么它就没有编译......

这是编译文件：

# Makefile for Movie Recommendation--- HW5 of Search Engine 11-641
#   Yitong Zhou
#   Nov 19, 2012
#source code directory:
src=src
#obj code directory:
obj=obj

recommend: $(obj)/main.o $(obj)/CorpusExp.o
    g++ -o $@ $<
$(obj)/main.o: src/main.cpp src/CorpusExp.cpp
    g++ -c $< -o $@
$(obj)/CorpusExp.o: $(src)/CorpusExp.cpp $(src)/CorpusExp.h
    g++ -c $< -o $@
clean:
    rm -rf obj/*.o recommend