C++ 实现模板 Singleton class 时出现链接错误

Question

I'm implementing a Singleton template in C++. I try to achieve thread-safe by std::call_once and std::once_flag, but somehow link error happens.

singleton.h

#ifndef _SINGLETON_H_
#define _SINGLETON_H_

#include <boost/noncopyable.hpp>
#include <mutex>

template<typename T>
class Singleton : boost::noncopyable {
public: 
    Singleton() = delete;

    static T& getInstance() {
        std::call_once(init_flag_, &Singleton::init);
        return *val_;
    }

private:
    static void init() {
        val_ = new T();
    }

private:
    static std::once_flag init_flag_;
    static T* val_; 
};

#endif // _SINGLETON_H_

test_singleton.cc

#include "singleton.h"
#include <iostream>

class Log {
public:
    void log() {
        std::cout << "log" << std::endl;
    }
};

int main() {
    Log & logger = Singleton<Log>::getInstance();
    logger.log();
}

And my g++ statement is

g++ -std=c++14 -pthread -o test test_singleton.cc

Error message:

/tmp/ccoxQBXl.o: In function `Singleton<Log>::getInstance()':
test_singleton.cc:(.text._ZN9SingletonI3LogE11getInstanceEv[_ZN9SingletonI3LogE11getInstanceEv]+0x2c): undefined reference to `Singleton<Log>::init_flag_'
test_singleton.cc:(.text._ZN9SingletonI3LogE11getInstanceEv[_ZN9SingletonI3LogE11getInstanceEv]+0x38): undefined reference to `Singleton<Log>::val_'
/tmp/ccoxQBXl.o: In function `Singleton<Log>::init()':
test_singleton.cc:(.text._ZN9SingletonI3LogE4initEv[_ZN9SingletonI3LogE4initEv]+0x11): undefined reference to `Singleton<Log>::val_'
collect2: error: ld returned 1 exit status

Answer 1

Finally got it. The only problem is I didn't initialize the static variables. For C++17, inline keyword allows initialization within class.

#ifndef SINGLETON_H_
#define SINGLETON_H_

#include <boost/noncopyable.hpp>
#include <mutex>

template<typename T>
class Singleton : boost::noncopyable {
public: 
    Singleton() = delete;

    static T& getInstance() {
        std::call_once(init_flag_, &Singleton::init);
        return *val_;
    }

private:
    static void init() {
        val_ = new T();
    }

private:
    static inline std::once_flag init_flag_{};
    static inline T* val_ = nullptr; 
};

#endif // SINGLETON_H_