工业强度的n吨基类模板

Question

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:

确保一个类只有n个实例,并提供对它们的全局访问点.

到目前为止,这是我的代码:

template<typename Derived, size_t n = 1>
class n_ton_base                 // Singletons are the default
{
    static Derived instances[n + (n == 0)];
                              // Zerotons are supported, too
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, 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？

Answer 1

正如Etienne Cordonnier指出的那样,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, useful for Singletons
    template<size_t i = 0>
    static Derived& get_instance()
    {
        static_assert(i < n, "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,因此不需要数组.

这也实现了线程安全的延迟初始化,而我不必对它做任何事情.太好了!

Answer 2

在全球范围内添加:

template<typename Derived, size_t n>
Derived n_ton_base<Derived, n>::instances[n + (n == 0)];

顺便说一句,std::array<>允许零大小的数组,所以你可能想要考虑它.