对于从未听说过“Schwarz Counter”的人,请参阅this wikibook页面。
我正在尝试用C ++ 11模板实现这个习惯用法,除非模板参数不是默认可构造的,否则一切正常。这是我的实现:(参见问题的代码注释)
#include <type_traits>
template<typename T>
class Single {
public:
static T& get() {
return reinterpret_cast<T&>(instance.storage);
}
private:
struct Instance {
enum { kSize = sizeof(T), kAlign = std::alignment_of<T>::value };
int counter;
typename std::aligned_storage<kSize, kAlign>::type storage;
template<typename... Arg>
Instance(Arg&&... param) {
if (counter++ == 0) {
new (&storage) T(param...);
}
}
~Instance() {
if (--counter == 0) {
reinterpret_cast<T&>(storage).~T();
}
}
};
static Instance instance;
};
// The following won't compile if T isn't default constructible.
// I want to get rid of the definition in this case, and let the
// users of this template to provide their own definitions.
// I'd like to keep the it when T is default constructible, because
// most of the time T is (according to my experience, most global
// objects are constructed without arguments, singletons are
// essentially global objects).
//
// So, basically I want something like this:
// #if std::is_default_constructible<T>::value
// template<typename T>
// ...;
// #endif
// Could this be done in C++11 ? If not, how about C++14 ?
template<typename T>
typename Single<T>::Instance Single<T>::instance;
BTW,如果该类是默认可构造的,则使用此模板定义单例非常简单,例如:
class Y {
friend class Single<Y>;
Y(int, int) { std::cout << "Y()\n"; }
~Y() { std::cout << "~Y()\n"; }
};
class X {
friend class Single<X>;
X() { std::cout << "X()\n"; }
~X() { std::cout << "~X()\n"; }
};
int main() {
auto& x = Single<X>::get();
// auto& g = Single<Y>::get(); // oops
return 0;
}
答案 0 :(得分:3)
创建两个类。一个用于默认的可构造T,另一个用于非。
如果默认可构造实例是默认可构造的,则将您的主实现分派为仅使用默认的可构造实例。
template<class T,class=void>
struct WhichSingle {
using type=Single<T>;
};
template<class T>
struct WhichSingle<T,typename std::enable_if<
!std::is_default_constructible<T>::value
>::type> {
using type=SingleNoDefault<T>;
};
template<class X>
using Bob = typename WhichSignle<X>::type;
和Bob<X>
可以是Single
或SingleNoDefault
。
可以做出更好的名字,并努力减少重复,但这只是笨拙的工作。