how to implement singleton class in cpp

singleton

cpp

Publish Date: 2018-01-22

Word Count: 437

Read Times: 3 Min

Read Count:

Intro

what singleton solve?

from stackoverflow here

Singletons solve one (and only one) problem.

Resource Contention.
If you have some resource that
(1) can only have a single instance, and
(2) you need to manage that single instance,
you need a singleton.

There aren’t many examples. A log file is the big one. You don’t want to just abandon a single log file. You want to flush, sync and close it properly. This is an example of a single shared resource that has to be managed.

It’s rare that you need a singleton. The reason they’re bad is that they feel like a global and they’re a fully paid up member of the GoF Design Patterns book.

When you think you need a global, you’re probably making a terrible design mistake.

local static object

see c-singleton-design-pattern

Actually, in C++ preferred way is local static object.

singleton pure

class Singleton
{
private:
    Singleton();

public:
    Singleton(Singleton const&) = delete;
    Singleton& operator=(Singleton const&) = delete;

    static Singleton& instance()
    {
      static Singleton INSTANCE;
      return INSTANCE;
    }
};

singleton with shared_ptr

class Singleton
{
private:
   Singleton();

public:
    Singleton(Singleton const&) = delete;
    Singleton& operator=(Singleton const&) = delete;

    static std::shared_ptr<Singleton> instance()
    {
        static std::shared_ptr<Singleton> s{new Singleton};
        return s;
    }
};

singleton usage

#define DISALLOW_COPY(TypeName) \
    TypeName(const TypeName&)

#define DISALLOW_ASSIGN(TypeName) \
    TypeName& operator=(const TypeName&)

#define DISALLOW_COPY_AND_ASSIGN(TypeName) \
    TypeName(const TypeName&);               \
    TypeName& operator=(const TypeName&)

class CSingleton
{
public:
    static CSingleton &GetInstance()
    {
        static CSingleton instance;
        return instance;
    }
    void DoSomething()
    {
        printf("void CSingleton::DoSomething() called.\n");
    }

private:
    CSingleton() {};
    DISALLOW_COPY_AND_ASSIGN(CSingleton);
};

// usage
CSingleton::GetInstance().DoSomething();    // OK

CSingleton& singleton = CSingleton::GetInstance(); // OK with reference
singleton.DoSomething();

CSingleton singleton = CSingleton::GetInstance(); // ERROR (copy constructor)

Example

config.h

#pragma once

class Config
{
public:
    static Config& GetInstance(std::string filename="./config.ini");
    ~Config();

private:
    Config(std::string filename);
    Config(const Config& ref) {}
    Config& operator =(const Config& ref) { return *this; }
};

config.cpp

#include "Config.h"

/*
static config instance will only be created once by calling Config::Config,
when program exit,static variable will be destoryed by calling Config::~Config.
*/

Config& Config::GetInstance(std::string filename)
{
    static Config instance(filename); 
    return instance;
}

Config::Config(std::string filename)
{
    std::cout << "[LOG] Config::Config count= "<<count << std::endl;
    // load config from filename 
    // ...
}

Config::~Config()
{
    std::cout << "[LOG] Config::~Config count= " << count << std::endl;
}

mysqldb.cpp

void MysqlDb::load_config(std::string filename)
{
    this->mysql_connection = Config::GetInstance(filename).MYSQL_CONNECTION; 
    this->mysql_username = Config::GetInstance(filename).MYSQL_USERNAME;
    this->mysql_password = Config::GetInstance(filename).MYSQL_PASSWORD;
    this->mysql_database = Config::GetInstance(filename).MYSQL_DATABASE;
    this->max_connection_pool_size = Config::GetInstance(filename).MAX_CONNECTION_POOL_SIZE;
}