const string & as function parameter

Hi all,
I got a compiler error when using "const string&" as function parameter,
while using "const string" results in success. The following is my code:

////////////////////////////////////a.h
#include <string>
class A
{
public:
    //...
        bool func(const char* c);
    bool func(const std::string& s);
    //...
}

/////////////////////////////////a.cpp
#include "a.h"
#include <iostream>
using namespace std;

//...

bool A::func(const char* c)
{
    //...
}

bool A::func(const string& s)
{
    return func(s.c_str());
}

////////////////////////////////main.cpp

#include "a.h"
#include <iostream>
#include <string>
using namespace std;

int main()
{
    string s= "...";
    A a;
    if (a.func(s)) //here is line 11
        cout << "succeed" << endl;
    else
        cout << "failed" << endl;
    cin.get();
    return 0;
}
//////////////////////////////End of my code

g++ outputs :
main.cpp:11: undefined reference to `A::func(std::string)'
But if I change the definition and declaration of "bool func(const
std::string& s)" to "bool func(const std::string s)",
everything is ok.

Anyone can tell me why? THANKS VERY MUCH!

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