c++ - std::tuple get() 成员函数

Question

boost::tuple 有一个 get() 成员函数，使用如下:

tuple<int, string, string> t(5, "foo", "bar");
cout << t.get<1>();  // outputs "foo"

看来C++0x std::tuple 没有这个成员函数，只能改用非成员函数形式:

std::get<1>(t);

在我看来更丑。

std::tuple 没有成员函数有什么特别的原因吗？还是只是我的实现(GCC 4.4)？

Answer 1

来自 C++0x 草案:

[ Note: The reason get is a nonmember function is that if this functionality had been provided as a member function, code where the type depended on a template parameter would have required using the template keyword. — end note ]

这可以用这段代码来说明:

template <typename T>
struct test
{
  T value;
  template <int ignored>
  T&  member_get ()
  {  return value;  }
};

template <int ignored, typename T>
T&  free_get (test <T>& x)
{  return x.value;  }

template <typename T>
void
bar ()
{
  test <T>  x;
  x.template member_get <0> ();  // template is required here
  free_get <0> (x);
};