Be Mindful with Compiler-Generated Move Constructors

Published on Jul 03, 2018.

So, I wrote a simple struct:

struct simple_struct
    std::function<void(int)> func;
    std::vector<int> data; // call func with each element in data

… and then had a vector of those:

std::vector<simple_struct> stuffs;

… and I was a happy (and oblivious) camper since everything worked.

If you spotted the problem by now, well done. For me it was a surprise. Curiously when I told the story to some friends, they were suprised too. Hopefully this blog post helps others who didn’t see the following coming.

After some time while debugging (and luckily not using Just My Code) I stepped into the copy-constructor of vector<int> while vector<simple_struct> was being resized.

“Whoa!”, I thought, “Why do you want to copy the struct? It’s obviously movable, so just use the compiler-generated move constructor.”

After a short investigation I found that the culprit was the std::function member because, as it turns out, std::function’s move constructor is not noexcept. This is a big problem because std::vector uses std::move_if_noexcept on reallocation to move its members only if their move constructors are noexcept and otherwise copy them. That’s because there is simply no reasonable way to preserve the valid state of the vector if a move throws an exception.

And why isn’t std::function’s move constructor noexcept? Well in hindsight it’s obvious. There is no guarantee that the variables captured by value from a lambda will have noexcept move constructors. std::function must be able to hold any lambda, so for that to be possible it can’t have a noexcept move constructor of its own.1

Finally, since a member’s move constructor isn’t noexcept, the compiler-generated move constructor of a class or struct is also not noexcept. It’s only logical.

So, it turns out that there were vectors of integers (and big ones, mind you) copied on each reallocation of the vector of simple_struct. A sad waste of CPU cycles.

Luckily the fix was simple. A = default move constructor is noexcept.2 I didn’t need to copy the struct, so I just deleted the copy constructor. Thus the vector had no choice but to use the move constructor. I did this with full realization that should a move constructor throw, the vector will end up in an invalid state. I documented it, but it’s highly unlikely for this to ever become an issue. So the final version of simple_struct, which wasn’t pointlessly copied, ended up like this:

struct simple_struct
    simple_struct() = default;
    simple_struct(simple_struct&&) = default;
    simple_struct(const simple_struct&) = delete;

    std::function<void(int)> func;
    std::vector<int> data; // call func with each element in data

So, what if you do need to copy that struct? In that case, you will sadly have to write your own noexcept move constructor and in it manually move all members (like a caveman).

And what if you do expect your objects to throw on move? Well… even sadlier, in that case, you will have to do something about it. Either don’t use std::vector, or don’t hold your objects by value, or something even smarter that I can’t think of. In the case of std::function you may have to resort to writing your own equivalent which doesn’t have a small buffer optimization and doesn’t have to move its contents (or at least not if they aren’t is_nothrow_move_constructible).

I’m not going to get into the argument on whether all move constructors should be noexcept. Always. Noexceptions. (but I think they should)

The sad truth is that we, as C++ programmers, in order to do our job must learn several rules… and then learn several thousand exceptions and special cases, so we don’t blow our whole legs off.

I added this to my mental list of gotchas and hopefully this blog post will help others to be more mindful when relying on the compiler-generated move constructor.

  1. As pointed out by comments on Cpplang Slack. This isn’t that obvious. std::function doesn’t have to move its contents. True, most if not all implementation use a small buffer optimization, but they may choose not to use it in case its contents don’t agree with is_nothrow_move_constructible. And in fact in the latest versions (after 2017) of libc++ and libstdc++ it is so! 

  2. Thanks to Miro Knejp for pointing out this error. 

Tags: c++

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