I am working on an n-ton base class template. I don't worry about laziness yet, so the Intent is:
Ensure a class has only n instances, and provide a global point of access to them.
Here is my code so far:
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];
}
};
And here is how one would use it:
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();
}
Unfortunately, the code doesn't compile yet:
/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
What did I do wrong?
As Etienne Cordonnier pointed out, it is much easier to use a local static instead of a class static:
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;
}
};
Note that each instantiated member function will have its own local static, so no array is needed.
This also achieves thread-safe lazy initialization without me having to do anything about it. Nice!
Add this at global scope:
template<typename Derived, size_t n>
Derived n_ton_base<Derived, n>::instances[n + (n == 0)];
Btw, std::array<>
allows zero-size arrays, so you might want to consider it.
You can find here an explanation about class static members:
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:
class X { public: static int i; }; int X::i = 0; // definition outside class declaration
Therefore you have to define n_ton::instances
outside of your class.
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