模板类

Question

我需要在模板类中初始化一个静态bool，我试着像this那样做。我能看到的唯一区别是我对类型参数T有一个约束，但这会导致编译错误，为什么？我该如何解决这个问题？

代码如下：

template<class T, class = typename enable_if<is_any_integral<T>::value>::type>
class fraction {
    static bool auto_reduce;

    // ...
};

template<class T, class = typename enable_if<is_any_integral<T>::value>::type>
bool fraction<T>::auto_reduce = true;

错误是：

  

错误：嵌套名称说明符＆＃39; fraction<T>::＆＃39;声明不引用类，类模板或类模板部分特化
  bool fraction<T>::auto_reduce = true;

Answer 1

Maybe simpler

template<class T, class V>
bool fraction<T, V>::auto_reduce = true;

When you write

template<class T, class = typename enable_if<is_any_integral<T>::value>::type>
class fraction

you say that fraction is a class with two type template paramenters; the std::enable_if if part is useful to assign a default value to the second parameter (and to permit the enable/not enable SFINAE works) but fraction is and remain a template class with two parameters, you have to cite both and there is no need to repeat the enable/not enable/default part for second parameter initializing auto_reduce.