工业强度的 n 吨基类模板

Industrial-strength n-ton base class template

本文关键字：基类      更新时间：2023-10-16

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：

确保类只有 n 个实例，并提供对它们的全局访问点。

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, 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，代码尚未编译：

/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？

正如 Etienne Cordonnier 指出的那样，使用局部静态比使用静态类要容易得多：

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函数，因此不需要数组。

这也实现了线程安全的延迟初始化，而无需我对此执行任何操作。好！

在全局范围内添加以下内容：

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，所以你可能要考虑一下。

你可以在这里找到一个关于类静态成员的解释：

静态数据成员（仅限C++）：

类的成员列表中静态数据成员的声明不是定义。必须在命名空间范围内定义类声明之外的静态成员。例如：

class X {
public:
    static int i;
};
int X::i = 0; // definition outside class declaration

因此，您必须在类之外定义n_ton::instances。

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