c++ - C++单例中的指针和非指针实例

Question

我在练习如何制作单例。一切都很顺利，直到我决定玩它。工作版本是:

唱.hp:

class GlobalClass
{
private:
    GlobalClass(int);
    int m_value;
    static GlobalClass *s_instance;
//    static GlobalClass instance;
  public:

    int get_value();
    void set_value(int v);
    static GlobalClass *GetS_instance();
//    static GlobalClass GetInstance();
};

唱.cpp

#include"sing.hpp"
#include<iostream>
using namespace std;

GlobalClass * GlobalClass::s_instance = 0;
//GlobalClass  GlobalClass::instance;

GlobalClass::GlobalClass(int v = 10)
{
    this->m_value = v;

}

int GlobalClass::get_value()
{
    return this->m_value;
}

void GlobalClass::set_value(int v)
{
    this->m_value = v;
}

GlobalClass * GlobalClass::GetS_instance()
{
    if (!s_instance)
    {
        s_instance = new GlobalClass;
    }
    return s_instance;
}

//GlobalClass  GlobalClass::GetInstance()
//{
//
//    return instance;
//}

主要.cpp:

#include "sing.hpp"
#include<iostream>
using namespace std;

void foo(void)
{
  GlobalClass::GetS_instance()->set_value(1);
  cout << "foo: global_ptr is " << GlobalClass::GetS_instance()->get_value() << '\n';
}

int main()
{
    cout << "main: global_ptr is " << GlobalClass::GetS_instance()->get_value() << '\n';
      foo();
      return 1;
}

在我取消注释之前，上面的设置运行良好:我收到以下错误:

~/workspace/singleton2$ g++ *.cpp -o dr
sing.cpp:6:27: error: no matching function for call to ‘GlobalClass::GlobalClass()’
sing.cpp:6:27: note: candidates are:
sing.hpp:5:2: note: GlobalClass::GlobalClass(int)
sing.hpp:5:2: note:   candidate expects 1 argument, 0 provided
sing.hpp:2:7: note: GlobalClass::GlobalClass(const GlobalClass&)
sing.hpp:2:7: note:   candidate expects 1 argument, 0 provided

除了不知道为什么会这样，奇怪的是:错误消息的第一行指向我定义实例的程序中的第 6 行，而错误表明我的构造函数中有错误。你能帮我解决这个问题吗？谢谢大家

Answer 1

您按值返回会生成一个拷贝，该拷贝不起作用，因为您不希望 Singelton 具有复制构造函数。

将其更改为通过引用返回:

GlobalClass  &  GlobalClass::GetInstance()
   //       ^^^
{
    return instance;
}

当你在这里时，你应该确保禁用复制构造函数和赋值运算符

更新你的默认参数在错误的地方。

GlobalClass::GlobalClass(int v = 10)
             //               ^^^^^ Remove this
{
    this->m_value = v;
}


private:
    GlobalClass(int v = 10);
            //      ^^^^^^   Add this

参见:C++ Singleton design pattern