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);
};
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.
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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