派生类中的显式模板静态成员实例化 [英] Explicit template static member instantiation in a derived class

问题描述

我试图用静态成员实现一个模板类。

I am trying to implement a template class with a static member. Classes that are derived from the template class shall be instantiated without the need to write extra code.

这是我天真的（而不是成功的）方法：

Here is my naive (and not successful) approach:

Singleton.h：

Singleton.h:

template <class T> class Singleton {
protected:
  Singleton();
  static T instance_;
}

// explicit instantiation of 'instance_' ???, 
// where 'instance_' is an instance of the derived class
template <class T> T Singleton<T>::instance_;

ConcreteA.h：

ConcreteA.h:

class ConcreteA : public Singleton<ConcreteA> {
public:
  ConcreteA();
  void foo();
}

main.c：

main.c:

int main() {
  // an instance of ConcreteA should have been created (no extra code)!!!
  return 0;
}

有办法强制实例化<$ c $通过从 Singleton 中导出 ConcreteA ，无需编写额外的实例化代码， ConcreteA / strong>

Is there a way to force the instantiation of ConcreteA by just deriving ConcreteA from Singleton, without writing extra instantiation code?

脏办法是调用实例_ ConcreteA 构造函数，例如：

A dirty workaround is to call an method on instance_ in the ConcreteA constructor, for example:

ConcreteA.c
$ b

ConcreteA.c

ConcrereA::ConcreteA { instance_.foo(); }

有更好的解决方法吗？

推荐答案

基于自己的诡计，这适用于我测试的所有编译器，并且不需要在派生类构造函数中的任何代码： p>

Building upon your own "dirty trick", this works in all the compilers I tested, and doesn't require any code in the derived class constructor:

#include <iostream>

template <class T> class Singleton {
protected:
  Singleton() { instptr_ = &instance_; }
  static T instance_;
private:
  static T* instptr_;
};

template<class T> T Singleton<T>::instance_;
template<class T> T* Singleton<T>::instptr_;

class ConcreteA : public Singleton<ConcreteA> {
public:
  ConcreteA() { std::cout << "ConcreteA constructed.\n"; }
  void foo();
};

int main() {
  //Prints 'ConcreteA constructed.'.
  return 0;
}

我的理解是取地址实例_ odr使用，强制它成立。我必须说，我不是100％肯定这是保证不会被优化在一些编译器的未来版本（我测试它与 -O2 无处不在）。

My understanding of it is that taking the address of instance_ odr-uses it, forcing it into existence. I must say I'm not 100% sure this is guaranteed not to be optimized away in future versions of some compiler (I tested it with -O2 everywhere).

编辑：看起来甚至写这样的基类构造函数

Looks like even writing the base class constructor like this

Singleton() { (void)&instance_; }

就足够了，可以除去 instptr _

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