C++ 11标准STL中Traits的is_pointer的实现

在看STL的源码，发现is_pointer的模板调用，写了一个测试代码如下：

#include <iostream>
#include <type_traits>

using namespace::std;

namespace iotek{
	template<typename _Tp, _Tp __v>
    struct integral_constant
    {
      static constexpr _Tp                  value = __v;
      typedef _Tp                           value_type;
      typedef integral_constant<_Tp, __v>   type;
      constexpr operator value_type() { return value; }
    };

	template<typename _Tp, _Tp __v>
    constexpr _Tp integral_constant<_Tp, __v>::value;

	/// The type used as a compile-time boolean with true value.
	typedef integral_constant<bool, true>     true_type;

	/// The type used as a compile-time boolean with false value.
	typedef integral_constant<bool, false>    false_type;

	template<typename>
		struct __is_pointer_helper
		: public false_type { };

	template<typename _Tp>
		struct __is_pointer_helper<_Tp*>
		: public true_type { };

	template<typename>
		struct remove_cv;

	  /// remove_const
	template<typename _Tp>
		struct remove_const
		{ typedef _Tp     type; };

	template<typename _Tp>
		struct remove_const<_Tp const>
		{ typedef _Tp     type; };

	/// remove_volatile
	template<typename _Tp>
		struct remove_volatile
		{ typedef _Tp     type; };

	template<typename _Tp>
		struct remove_volatile<_Tp volatile>
		{ typedef _Tp     type; };

	/// remove_cv
	template<typename _Tp>
    struct remove_cv
    {
      typedef typename
      remove_const<typename remove_volatile<_Tp>::type>::type     type;
    };

	/// is_pointer
	template<typename _Tp>
    struct is_pointer
    //: public integral_constant<bool, (__is_pointer_helper<typename remove_cv<_Tp>::type>::value)>
	: public integral_constant<bool, (__is_pointer_helper<_Tp>::value)>
    { };

	/*
	public integral_const<bool,(_is_pointer_helper<remove_cv<int>::type>::value)>

	*/

	template <typename T>
	void foo(const T& val)
	{
		if(is_pointer<T>::value){
			cout << "foo() called for a pointer " << endl;
		}else{
			cout << "foo() called for a value" << endl;
		}
	}

}

using namespace::iotek;

int main(int argc, char*argv[])
{
	int i = 0;
	foo(&i);
	return 0;
}

编译使用如下命令：

gcc -std=c++11 -o test is_pointer.cpp