c++ Compile error - Undefined symbols for architecture x86_64
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/5818270/undefined-symbols-for-architecture-i386">Undefined symbols for architecture i386:</a>  </p>
</blockquote>



<pre><code>using namespace std;
#include &lt;iostream&gt;
#include &lt;fstream&gt;
#include &lt;string&gt;
#include "CorpusExp.h"

int main(int argc, char* argv[]){
ifstream infile("../DATA.txt");
string train_dir;   // The training data directory
infile&gt;&gt;train_dir;
train_dir=train_dir.substr(6,train_dir.length());

if (argc&gt;1){
    if (strcmp(argv[1],"-s")){ // enter into CorpusExploration mode
        CorpusExp ce(train_dir,"all");   //&lt;&lt;=======LINE X!!!!!!!!
        //ce.calculate();
        if (argc&gt;=3 &amp;&amp; strcmp(argv[2],"-u")){ // check user stats
            cout&lt;&lt;"shit";
        }
        else if (argc&gt;=3 &amp;&amp; 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;
}
</code></pre>

<p><strong>If I include the LINE X, the main.cpp won't compile, the error message is:</strong></p>

<pre><code>      Undefined symbols for architecture x86_64:
   "CorpusExp::CorpusExp(std::basic_string&lt;char, std::char_traits&lt;char&gt;,     std::allocator&lt;char&gt; &gt; const&amp;, std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt; const&amp;)", 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
</code></pre>

<p><strong>And here is my CorpusExp.cpp and CorpusExp.h:</strong></p>

<p>/* CorpusExporater header file
   Used for statistical purpose</p>

<p>*/</p>

<pre><code>using namespace std;
#include &lt;iostream&gt;
#include &lt;fstream&gt;
#include &lt;string&gt;
#include &lt;dirent.h&gt;  // the Linux library to read the directories

class CorpusExp{
public:
    CorpusExp(const string&amp;,const string&amp;); // (dir_name, mode)
    void calculate();
    void display(ostream &amp;out, const int&amp;);    // 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 &amp;dir_name, const string &amp;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&lt;&lt;epdf-&gt;d_name&lt;&lt;endl;
        }
    }
}
void CorpusExp::display(ostream &amp;out, const int &amp;id){

}
</code></pre>

<p><strong>I am barely new to c++, so I get very confused, don't know which part caused this problem. If I remove the LINE X, everything will just be fine, but if I declare the CorpusExp class in the main function, then it did not compile......</strong></p>

<p>here is the compile file:</p>

<pre><code># 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 $@ $&lt;
$(obj)/main.o: src/main.cpp src/CorpusExp.cpp
    g++ -c $&lt; -o $@
$(obj)/CorpusExp.o: $(src)/CorpusExp.cpp $(src)/CorpusExp.h
    g++ -c $&lt; -o $@

clean:
    rm -rf obj/*.o recommend
</code></pre>
