std::mem_fun vs std::mem_fn

c++, c++11, member-functions, std

Solution

`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.

Problem

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