std::tuple get() 成员函数

问题描述

boost::tuple 有一个 get() 成员函数，像这样使用:

boost::tuple has a get() member function used like this:

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

似乎C++0x std::tuple 没有这个成员函数，你必须改用非成员函数形式:

It seems the C++0x std::tuple does not have this member function, and you have to instead use the non-member function form:

std::get<1>(t);

在我看来哪个更丑.

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)?

Is there any particular reason why std::tuple doesn't have the member function? Or is it just my implementation (GCC 4.4)?

推荐答案

来自 C++0x 草案:

From C++0x draft:

[ 注意:get 是非成员函数的原因是，如果此功能已作为成员函数提供，则类型依赖于模板参数的代码将需要使用模板关键字.— 尾注 ]

[ 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 ]

这可以用以下代码说明:

This can be illustrated with this code:

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);
};

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