关于c ++:工业级n吨基类模板

Industrial-strength n-ton base class template

我正在开发一个n-ton基类模板。我还不担心懒惰,所以目的是:

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

这是迄今为止我的代码:

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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];
    }
};

下面是如何使用它:

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class SingletonExample : public n_ton_base<SingletonExample>
{
public:
    void method()
    {
        std::cout <<"Singletons are overused.
"
;
    }    
};

class DoubletonExample : public n_ton_base<DoubletonExample, 2>
{
public:
    void method()
    {
        std::cout <<"Doubleton" << this <<" says hello.
"
;
    }    
};

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:

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/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EtienneCordonnier指出的,使用局部静态而不是类静态更容易:

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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;
    }
};

注意,每个实例化的成员函数都有自己的局部静态,因此不需要数组。

这还实现了线程安全的延迟初始化,而不需要我做任何事情。好极了!


在全局范围中添加:

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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,所以您可能会考虑它。


您可以在这里找到关于类静态成员的解释:

Static data members (C++ only):

The declaration of a static data member in the member list of a class is not a definition. You must define the static member outside of the class declaration, in namespace scope. For example:

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class X {
public:
    static int i;
};

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

因此,您必须在类外定义n_ton::instances