如何在单例中传递参数

Question

我一直想知道如何将参数传递给单例构造函数。我已经知道如何做单例了，但是我很不幸地找到了一种方法。

这是我的代码（部分）。

Questionnary* Questionnary::getInstance(){

    static Questionnary *questionnary = NULL;

    if(questionnary == NULL){
        cout << "Object created";
        questionnary = new Questionnary();

    }
    else if(questionnary != NULL){
        cout << "Object exist";
    }

    return questionnary;
}

Questionnary::Questionnary(){
    cout << "I am an object";
}

//This is want i want to acheive
Questionnary::Questionnary(string name){
    cout << "My name is << name;
}

在此先感谢

（顺便说一句，我知道如何以及为什么单身不好）

Answer 1

您不需要动态分配单例的实例。它可能看起来像以下方式（这有时被称为“延迟加载单例”~实例创建较晚并且有点“自动”）：

#include <iostream>
#include <string>

class Questionnary
{
private:
    // constructor taking string:
    Questionnary(const std::string& name) : name_(name) { }
public:
    static Questionnary& getInstance(const std::string& name)
    {
        static Questionnary q(name);
        std::cout << "My name is: " << q.name_ << std::endl;
        return q;
    }
private:
    std::string name_;
};

int main() {
    Questionnary::getInstance("Josh");
    Questionnary::getInstance("Harry");
}

输出：

My name is: Josh
My name is: Josh

请注意，当第一次调用getInstance 时，构造函数只会被调用一次。

Answer 2

让我为您的用例扩展Martin York's answer。我建议在这种特殊情况下使用指针作为参数，因为我们利用它的固有属性，它可以是“空的”。

class Questionnary
{
  std::string _str;

  static Questionnary& getInstanceImpl(std::string* const s = nullptr)
  {
    static Questionnary instance{ s };
    return instance;
  }

  Questionnary(std::string* const s)
    : _str{ s ? move(*s) : std::string{} } // employ move ctor
  {
    if (nullptr == s)
      throw std::runtime_error{ "Questionnary not initialized" };
  }

public:
  static Questionnary& getInstance()
  {
    return getInstanceImpl();
  }
  static void init(std::string s) // enable moving in
  {
    getInstanceImpl(&s);
  }

  Questionnary(Questionnary const&) = delete;
  void operator=(Questionnary const&) = delete;
};

我发现这种方法不那么令人困惑，因为它让您在第一次初始化后获得实例而无需（无论如何丢弃）参数：

// first init
Questionnary::init("my single Questionnary");

// later on ...
Questionnary& q = Questionnary::getInstance();

编辑：从 getInstance 函数中删除了参数并进行了一些优化。

Answer 3

有一个方法来创建实例以将参数传递给构造函数，如果在调用它之前没有调用 CreateInstance，您可以在 getInstance() 方法中断言。喜欢：

class Questionnary
{
private:
    // constructor taking string:
    Questionnary(const std::string& name) : name_(name) 
    {
        std::cout << "My name is: " << q.name_ << std::endl; 
    }

    static Questionnary* m_instance;
public:
    static void createInstance(const std::string& name)
    {
        assert(!m_instance);
        m_instance = new Questionary(name);
    }

    static void destroyInstance()
    {
        assert(m_instance);
        delete m_instance;
    }

    static Questionnary* Questionnary::getInstance()
    {
        assert(m_instance);
        return m_instance;
    }
private:
    std::string name_;
};

Answer 4

我的版本使用现代 C++，包装现有类型：

#ifndef SINGLETON_H
#define SINGLETON_H

template <typename C, typename ...Args>
class singleton
{
private:
  singleton() = default;
  static C* m_instance;

public:
  ~singleton()
  {
    delete m_instance;
    m_instance = nullptr;
  }
  static C& instance(Args...args)
  {
    if (m_instance == nullptr)
      m_instance = new C(args...);
    return *m_instance;
  }
};

template <typename C, typename ...Args>
C* singleton<C, Args...>::m_instance = nullptr;

#endif // SINGLETON_H

这是单元测试中的样子：

  int &i = singleton<int, int>::instance(1);
  UTEST_CHECK(i == 1);

  tester1& t1 = singleton<tester1, int>::instance(1);
  UTEST_CHECK(t1.result() == 1);

  tester2& t2 = singleton<tester2, int, int>::instance(1, 2);
  UTEST_CHECK(t2.result() == 3);

问题在于 instance() 在每次调用时都需要参数，但只在第一次调用时使用它们（如上所述）。一般不能使用默认参数。最好使用 create(Args...) 方法，该方法必须在调用 instance() 或引发异常之前。

Answer 5

//! @file singleton.h
//!
//! @brief Variadic template to make a singleton out of an ordinary type.
//!
//! This template makes a singleton out of a type without a default
//! constructor.

#ifndef SINGLETON_H
#define SINGLETON_H

#include <stdexcept>

template <typename C, typename ...Args>
class singleton
{
private:
  singleton() = default;
  static C* m_instance;

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

  ~singleton()
  {
    delete m_instance;
    m_instance = nullptr;
  }

  static C& create(Args...args)
  {
    if (m_instance != nullptr)
      {
    delete m_instance;
    m_instance = nullptr;
      }
    m_instance = new C(args...);
    return *m_instance;
  }

  static C& instance()
  {
    if (m_instance == nullptr)
      throw std::logic_error(
        "singleton<>::create(...) must precede singleton<>::instance()");
    return *m_instance;
  }
};

template <typename C, typename ...Args>
C* singleton<C, Args...>::m_instance = nullptr;

#endif // SINGLETON_H

和：

void
singleton_utest::test()
{
  try
    {
      singleton<int, int>::instance();
      UTEST_CHECK(false);
    }
  catch (std::logic_error& e)
    {
      UTEST_CHECK(true);
    }

  try
    {
      UTEST_CHECK((singleton<int, int>::create(1) == 1));
      UTEST_CHECK((singleton<int, int>::instance() == 1));
    }
  catch (...)
    {
      UTEST_CHECK(false);      
    }

  using stester0 = singleton<tester0>;

  try
    {
      stester0::instance();
      UTEST_CHECK(false);
    }
  catch (std::logic_error& e)
    {
      UTEST_CHECK(true);
    }

  try
    {
      UTEST_CHECK((stester0::create().result() == 0));
      UTEST_CHECK((stester0::instance().result() == 0));
    }
  catch (...)
    {
      UTEST_CHECK(false);      
    }

  using stester1 = singleton<tester1, int>;

  try
    {
      stester1::instance();
      UTEST_CHECK(false);
    }
  catch (std::logic_error& e)
    {
      UTEST_CHECK(true);
    }

  try
    {
      UTEST_CHECK((stester1::create(1).result() == 1));
      UTEST_CHECK((stester1::instance().result() == 1));
    }
  catch (...)
    {
      UTEST_CHECK(false);      
    }

  using stester2 = singleton<tester2, int, int>;

  try
    {
      stester2::instance();
      UTEST_CHECK(false);
    }
  catch (std::logic_error& e)
    {
      UTEST_CHECK(true);
    }

  try
    {
      UTEST_CHECK((stester2::create(1, 2).result() == 3));
      UTEST_CHECK((stester2::instance().result() == 3));
    }
  catch (...)
    {
      UTEST_CHECK(false);      
    }
}