std::tuple get() member function

Question

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

tuple t(5, \"foo\", \"bar\");
cout << t.g         


        

Answer 1

The existing answers are great and certainly for the standards committee were vital for this purpose. But there is another issue that I think is important enough to mention.

With free functions, you have the ability to modify an interface without changing the definition of a class. You can make any type "gettable" simply by specializing the global get. With a member function, you would have to directly modify the class.

Range-based for looks for member begin/end on class types, but it also looks for non-member begin/end via ADL. These APIs can be used with any container, even those that don't have begin/end functions. You can specialize it for, for example, LibXML2 element types, such that you can range-based for over xmlElement*'s.

You can't do that if they had to be member functions.

In C++, free functions are a natural interface for many operations that could be done on many different kinds of classes.

Answer 2

From C++0x draft:

[ 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 ]

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);
};

Answer 3

N3090/3092, §20.4.2.6/8: "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"