我正在研究一个n吨的基类模板。我现在不用担心懒惰,所以目的是:
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];
}
};
这是一种使用方式:
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
我做错什么了?
最佳答案
作为Etienne Cordonnier pointed out,使用局部静态变量而不是类静态变量要容易得多:
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将具有其自己的局部静态变量,因此不需要数组。
这也可以实现thread-safe lazy initialization,而无需我做任何事情。好的!