What is the difference between std::mem_fun
and std::mem_fn
? Why is the naming so confusing?
Boost's documentation says that std::mem_fn
can replace std::mem_fun
in most cases. So in what situation would you still use std::mem_fun
?
std::mem_fun
is deprecated. std::mem_fn
can do everything it does, and it does it more conveniently. The relation between the two is the same as the relation between std::bind1st
/std::bind2nd
and the C++11 std::bind
. Both std::mem_fn
and std::bind
were developed and mastered after std::bind1st
and std::mem_fun
were made into the C++98 Standard. So that means we had to wait until C++11 to properly replace the old stuff with the superior alternatives.
For instance, std::mem_fun
can only deal with member functions that take one or no argument. std::mem_fn
is variadic and can deal with members that take any number of arguments.
You also need to pick between std::mem_fun
and std::mem_fun_ref
depending on whether you want to deal with pointers or references for the class object (respectively). std::mem_fn
alone can deal with either, and even provides support for smart pointers.
The documentation of boost::mem_fn
explains when to use std::mem_fun
, and put simply that's when you need to operate with code that expects std::mem_fun
, or that expects adaptable functors (which is an obsolete notion* from C++03). For those cases you wouldn't be able to plug in std::mem_fn
either, so there you have it: you would use std::mem_fun
for legacy.
*: I mean by that that new code shouldn't rely on the C++03 protocol of having e.g. result_type
member types (it's more customary to use the new traits like std::result_of
) -- the new facilities like std::bind
/std::mem_fn
do in fact provide those members if they would have been present in equivalent C++03 code. I leave it to you to figure out whether you should update old code that relies on adaptable functors with std::mem_fn
by relying on this behaviour.