std::tuple get() member function

Question

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

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

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);

which to me looks uglier.

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

As a design rule of thumb, non-member functions are to be preferred anyway. — GManNickG
– GManNickG, Commented Jul 22, 2010 at 23:59

user319799 · Accepted Answer · 2010-07-22 21:26:44Z

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); };

I see. Still, they could have provided both member and non-member functions like boost, if for no other reason that to be compatible with boost (since a lot of people are likely to have already been using boost tuples before switching to C++0x).
@HighCommander4 Whereas I understand your argument, boost compatibility alone is not a sufficient reason on its own, since compatibility to existing solutions shouldn't be the only driving force for providing something in a new standard and even boost is not the non-plus-ultra and may profit from some revision. Those general words being said, I still agree that member-gets would have been a good idea.
Now that we have C++11 that no longer requiring template keyword, we can totally have a member function now? Just a proposal needed?

Nicol Bolas · Accepted Answer · 2018-07-30 13:48:44Z

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.

std::begin/std::end are also good examples of how member and non-member functions can work together nicely. You can use either with most containers.
Ok this is cool and all but provide the member function too!!

Jerry Coffin · Accepted Answer · 2010-07-22 21:25:35Z

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"

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