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    <p><code>std::mem_fun</code> is deprecated. <code>std::mem_fn</code> can do everything it does, and it does it more conveniently. The relation between the two is the same as the relation between <code>std::bind1st</code>/<code>std::bind2nd</code> and the C++11 <code>std::bind</code>. Both <code>std::mem_fn</code> and <code>std::bind</code> were developed and mastered after <code>std::bind1st</code> and <code>std::mem_fun</code> 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.</p> <p>For instance, <code>std::mem_fun</code> can only deal with member functions that take one or no argument. <code>std::mem_fn</code> is variadic and can deal with members that take any number of arguments.</p> <p>You also need to pick between <code>std::mem_fun</code> and <code>std::mem_fun_ref</code> depending on whether you want to deal with pointers or references for the class object (respectively). <code>std::mem_fn</code> alone can deal with either, and even provides support for smart pointers.</p> <p>The documentation of <code>boost::mem_fn</code> explains when to use <code>std::mem_fun</code>, and put simply that's when you need to operate with code that expects <code>std::mem_fun</code>, or that expects adaptable functors (which is an obsolete notion* from C++03). For those cases you wouldn't be able to plug in <code>std::mem_fn</code> either, so there you have it: you would use <code>std::mem_fun</code> for legacy.</p> <p>*: I mean by that that new code shouldn't rely on the C++03 protocol of having e.g. <code>result_type</code> member types (it's more customary to use the new traits like <code>std::result_of</code>) -- the new facilities like <code>std::bind</code>/<code>std::mem_fn</code> 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 <code>std::mem_fn</code> by relying on this behaviour.</p>
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    1. CO`std::not1` and `std::not2` still expect those "adaptable" typedefs.
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    2. CO@Cubbi: Are they the only parts of C++11 that still do?
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    3. CO@Scotty as far as I've seen, yes, the only non-deprecated parts. And there are some 25 ways to create functors with those typedefs, from `std::function` and `std::mem_fn` to `std::ref` and `std::map::value_comp`.
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