c++ - 对 Singleton::instance 的未定义引用

Question

我正在尝试编写单例。当我编译它时，我得到了许多未定义的错误，比如instance和mutex_getSingleton()

对“Singleton::instance”的
未定义引用 对“Singleton::mutex_”的未定义引用

#include<iostream>
#include<string>
#include<mutex>
using namespace std;
class Singleton{
public:
    static Singleton* getSingleton(){
        if(instance == NULL){
            mutex_.lock();
            if(instance == NULL){
                instance = new Singleton();
            }
            mutex_.unlock();
        }
        return instance;
    }
private:
    Singleton(){}
    Singleton& operator =(const Singleton& ){}
    static Singleton *instance;
    static mutex mutex_;
};
int main(){
    Singleton* singleton = Singleton::getSingleton();
    return 0;
}

标签： c++singleton

Answer 1

您应该像那里一样将这个静态字段定义到 *.cpp 文件中

class Singleton{
public:
    static Singleton* getSingleton(){
        if(instance == NULL){
            mutex_.lock();
            if(instance == NULL){
                instance = new Singleton();
            }
            mutex_.unlock();
        }
        return instance;
    }
private:
    Singleton(){}
    Singleton& operator =(const Singleton& ){}
    static Singleton *instance;
    static mutex mutex_;
};

mutex Singleton::mutex_;
Singleton * Singleton::instance;

如果您使用单例 myers 和 c++11，则可以删除互斥锁：

class Singleton {
public:
    static Singleton& Instance() {
        static Singleton S;
        return S;
    }

private:
    Singleton();
    ~Singleton();
};