7

我正在研究一个 n-ton 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？

4

3 回答 3

9

正如Etienne Cordonnier 指出的那样，使用本地静态而不是类静态要容易得多：

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，因此不需要数组。

这也实现了线程安全的延迟初始化，而我无需对此做任何事情。好的！

于 2013-02-02T15:25:30.910 回答
8

在全局范围内添加：

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，因此您可能需要考虑它。

于 2013-02-02T14:11:18.850 回答
1

您可以在此处找到有关类静态成员的说明：

静态数据成员（仅限 C++）：

在类的成员列表中声明静态数据成员不是定义。您必须在命名空间范围内的类声明之外定义静态成员。例如：

class X {
public:
    static int i;
};

int X::i = 0; // definition outside class declaration

因此，您必须n_ton::instances在班级之外进行定义。

于 2013-02-02T14:16:11.063 回答