c – 工业强度的n吨基类模板

概述我正在研究一个n吨基类模板.我还不担心懒惰,所以意图是： Ensure a class has only n instances, and provide a global point of access to them. 这是我到目前为止的代码： template<typename Derived, size_t n = 1>class n_ton_base / 我正在研究一个n吨基类模板.我还不担心懒惰,所以意图是：

Ensure a class has only n instances,and provIDe a global point of access to them.

这是我到目前为止的代码：

template<typename Derived,size_t n = 1>class n_ton_base                 // Singletons are the default{    static Derived instances[n + (n == 0)];                              // Zerotons are supported,tooprotected:    // Prevent n_ton_base to be used outsIDe of inheritance hIErarchIEs    n_ton_base() {}     // Prevent n_ton_base (and Derived classes) from being copIEd    n_ton_base(const n_ton_base&) = delete;public:                   // Get first element by default,useful for Singletons    template<size_t i = 0>    static Derived& get_instance()    {        static_assert(i < n,"Time to increase n it seems!");        return instances[i];    }};

以下是人们如何使用它：

class SingletonExample : public n_ton_base<SingletonExample>{public:    voID method()    {        std::cout << "Singletons are overused.\n";    }    };class DoubletonExample : public n_ton_base<DoubletonExample,2>{public:    voID method()    {        std::cout << "Doubleton " << this << " says hello.\n";    }    };int main(){    SingletonExample::get_instance().method();    DoubletonExample::get_instance().method();    DoubletonExample::get_instance<0>().method();    DoubletonExample::get_instance<1>().method();}

不幸的是,代码还没有编译：

/tmp/ccsFtliS.o: In function `SingletonExample& n_ton_base<SingletonExample,1ul>::get_instance<0ul>()':nton.cpp:(.text._ZN10n_ton_baseI16SingletonExampleLm1EE12get_instanceILm0EEERS0_v[SingletonExample& n_ton_base<SingletonExample,1ul>::get_instance<0ul>()]+0x5): undefined reference to `n_ton_base<SingletonExample,1ul>::instances'/tmp/ccsFtliS.o: In function `DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<0ul>()':nton.cpp:(.text._ZN10n_ton_baseI16DoubletonExampleLm2EE12get_instanceILm0EEERS0_v[DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<0ul>()]+0x5): undefined reference to `n_ton_base<DoubletonExample,2ul>::instances'/tmp/ccsFtliS.o: In function `DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<1ul>()':nton.cpp:(.text._ZN10n_ton_baseI16DoubletonExampleLm2EE12get_instanceILm1EEERS0_v[DoubletonExample& n_ton_base<DoubletonExample,2ul>::get_instance<1ul>()]+0x5): undefined reference to `n_ton_base<DoubletonExample,2ul>::instances'collect2: ld gab 1 als Ende-Status zurück

我做错了什么？

解决方法 作为 Etienne Cordonnier pointed out,使用本地静态而不是静态类更容易：

template<typename Derived,size_t n = 1>class n_ton_base                 // Singletons are the default{protected:    // Prevent n_ton_base to be used outsIDe of inheritance hIErarchIEs    n_ton_base() {}     // Prevent n_ton_base (and Derived classes) from being copIEd    n_ton_base(const n_ton_base&) = delete;public:                   // Get first element by default,"Time to increase n it seems!");        static Derived instance;        return instance;    }};

请注意,每个实例化的成员函数都有自己的本地静态,因此不需要任何数组.

这也实现了thread-safe lazy initialization,而我不必对此做任何事情.太好了！

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