这是类头文件：

#ifndef _NETWORK_H_
#define _NETWORK_H_

#include <iostream>
#include <list>
#include "Module.h"

using namespace std;

/*
 * Assume only one network can run at a time 
 * in the program. So make the class a singleton.
 */
class Network {
private:
    static Network* _instance;
    list<Module*> _network;

public:
    static Network* instance();
};

#endif

这是impl文件：

#include "Network.h"

Network* Network::instance() {
    if (!_instance)
        _instance = new Network();
    return _instance;
}

这是编译器错误：

Undefined symbols for architecture x86_64:
  "Network::_instance",referenced from:
      Network::instance() in Network.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command Failed with exit code 1 (use -v to see invocation)