C++ 学习笔记
C++ 中单例模式的实现
简介
c++ 类 有复制构造函数，今天来介绍一些如何让一个类没有办法被 copy
普通的类, 可以通过复制构造函数类进行 copy
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16class Copyable
{
};
int main()
{
Copyable c1;
Copyable c2 = c1;
Copyable c3(c1);
Copyable& ref = c1;
Copyable c4 = ref;
Copyable c5(ref);
return 0;
}删除一些 拷贝构造函数, 这样就不可以被 copy 了
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class Noncopyable
{
public:
Noncopyable() = default;
Noncopyable(const Noncopyable&) = delete;
Noncopyable& operator=(const Noncopyable&) = delete;
Noncopyable(Noncopyable&&) = delete;
Noncopyable& operator=(Noncopyable&&) = delete;
};
class Copyable
{
};
int main()
{
Copyable c1;
Copyable c2 = c1;
Copyable c3(c1);
Copyable& ref = c1;
Copyable c4 = ref;
Copyable c5(ref);
// -----------------
Noncopyable n1;
Noncopyable n2 = n1; // error
Noncopyable n3(n1); // error
Noncopyable& n_ref = n1;
Noncopyable n4 = n_ref; // error
Noncopyable n5(n_ref); // error
return 0;
}
一些骚操作
借助于上面的例子，可以实现单个例模式
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class Noncopyable
{
public:
Noncopyable() = default;
Noncopyable(const Noncopyable&) = delete;
Noncopyable& operator=(const Noncopyable&) = delete;
Noncopyable(Noncopyable&&) = delete;
Noncopyable& operator=(Noncopyable&&) = delete;
};
template<typename T>
class SingleTon : public Noncopyable
{
public:
static T* instance()
{
std::call_once(once_, &SingleTon<T>::Init);
return instance_;
}
static T& get_instance()
{
return *instance();
}
static void ShutDown()
{
delete instance_;
}
private:
static void Init()
{
instance_ = new T();
}
static std::once_flag once_;
static T* instance_;
};
template<typename T>
std::once_flag SingleTon<T>::once_;
template<typename T>
T* SingleTon<T>::instance_ = nullptr;
// 单例
class SingleDemo final : public SingleTon<SingleDemo>
{
public:
int data;
};
int main()
{
SingleDemo* demo1 = SingleDemo::instance();
demo1->data = 1;
std::cout << demo1->data << std::endl;
demo1->data = 2;
SingleDemo* demo2 = SingleDemo::instance();
std::cout << demo1->data << std::endl;
return 0;
}上面这个例子的输出结果为
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