在模板初始化过程中,模板参数列表中的false的计算结果是什么[[副本]

this tutorial 基本上是有道理的。但我不知道代码的关键部分在哪里。

一秒钟之内 enable_if false 和模板类型 T 争论。

template <class T>
struct enable_if<false, T>
{};

为什么 这里用的是什么?当代码中遇到模板实例化时,false的计算结果是什么?如果有一个这样的模板参数类型的术语,我会很高兴如果你能分享这个术语。

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

template <typename T>
class is_container
{
  typedef char true_type;
  struct false_type { true_type _[2]; }; //false_type must be larger then the true_type. 
  template <typename U>
  static true_type has_iterator_checker(typename U::iterator *); //overload 1,.called if the type passed contains an iterator.  and returns a true type.
  template <typename U>
  static false_type has_iterator_checker(...); //if false type is passed //overload 2, called if the type passed DOESN'T contain an iterator. and returns a false type.
public:
  enum
  {
    value = (sizeof(has_iterator_checker<T>(0)) == sizeof(true_type)) //compares the size of the overlad with the true type. if they dont match, value becames false.
  };
};

template <bool Cond, class T = void> //first param is is a bool condition, second param is the return type. 
struct enable_if
{
  typedef T type; //if bool Cond is true, it returns the type.
};

//if bool cond is false, appereantly the following template does nothing and compiler silently fails without throwing error.  
//it seems this is what makes SFINAE a powerfull tool.
//but what i dont understand is that. <false, T>. there is no bool type before false. so what does this false evaluate into whenever compiler
//encounters  it the code?
template <class T>
struct enable_if<false, T>
{};

template <typename T>
typename enable_if<!is_container<T>::value>::type
super_print(T const &t)
{
  std::cout << t << std::endl;
}

template <typename T>
typename enable_if<is_container<T>::value>::type
super_print(T const &t)
{
  typedef typename T::value_type value_type;
  std::copy(t.begin(),
    t.end(),
    std::ostream_iterator<value_type>(std::cout, ", "));
  std::cout << std::endl;
}

int main()
{
  super_print(10); // a condition that is false.
  std::vector<int> b;
  b.push_back(1);
  b.push_back(2);
  b.push_back(3);
  super_print(b);

  return 0;
}