elasticsearch按字符串字段的长度过滤(elasticsearch filter by length of a string field)

 这是另一种解决方法，当您需要对某个函数进行部分特化时（这是不允许的）也很有用。 创建一个模板仿函数类（即，其唯一目的是执行单个成员函数的类，通常名为operator（）），对其进行特化，然后在模板函数中调用。 
 
 我想我从Herb Sutter那里学到了这个技巧，但是不记得是哪本书（或文章）。 根据您的需要，它可能有点过分，但仍然...... 
 
template <typename T>
struct select;

template <bool B>
struct testme_helper
{
  void operator()();
};

template <typename T>
class A
{
private:
  template <bool B> void testme()
  {
    testme_helper<B>()();
  }

public:
  void test()
  {
    testme<select<T>::value>();
  }
};

template<> void testme_helper<true>::operator()()
{
  std::cout << "true" << std::endl;
}

template<> void testme_helper<false>::operator()()
{
  std::cout << "false" << std::endl;
}

Here is another workaround, also useful when you need to partialy specialize a function (which is not allowed). Create a template functor class (ie. class whose sole purpose is to execute a single member function, usually named operator() ), specialize it and then call from within your template function.
 
I think I learned this trick from Herb Sutter, but do not remember which book (or article) was that. For your needs it is probably overkill, but nonetheless ...
 
template <typename T>
struct select;

template <bool B>
struct testme_helper
{
  void operator()();
};

template <typename T>
class A
{
private:
  template <bool B> void testme()
  {
    testme_helper<B>()();
  }

public:
  void test()
  {
    testme<select<T>::value>();
  }
};

template<> void testme_helper<true>::operator()()
{
  std::cout << "true" << std::endl;
}

template<> void testme_helper<false>::operator()()
{
  std::cout << "false" << std::endl;
}