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,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！
