proper way to swap unions?

Question

I have a class which has a union as one of its members. I also am a big fan of the copy/swap idiom. It occurred to me that there doesn't appear to be any correct (in the sense of the standard) to swap unions!

Here's the best that I could come up with:

char tmp[sizeof(U)];
memcpy(tmp, &u1, sizeof(U));
memcpy(&u1, &u2, sizeof(U));
memcpy(&u2, tmp, sizeof(U));

Since unions (at least in c++03) require that all members be POD, I don't see why this wouldn't work. But it just doesn't feel right. Is there a more correct way to swap unions? It seems almost like something that was overlooked.

What do you guys think?

EDIT:

OK, I feel kinda dumb after seeing the solutions given :-). I wrote off traditional solutions like std::swap and assignment because when I first wrote the code, the union was an anonymous union. In the current version of the code, the union is no longer anonymous and traditional solutions seem to work just fine. Thanks.

@DShook: while that is generally the case, there are some cases where they make sense. — Evan Teran, Apr 13 '11 at 15:59
If it's C++ only, maybe you want to implement a copy constructor on your struct, this way, you don't have to rely on a tmp array — Bruce, Apr 13 '11 at 16:03
Related question http://stackoverflow.com/questions/2466486/c-union-assignment-is-there-a-good-way-to-do-this — Robᵩ, Apr 13 '11 at 16:04
@Rob Adams: I think i wrote that off because some compilers (Visual Studio I think) didn't like that. — Evan Teran, Apr 13 '11 at 16:14
This will break in C++11, as unions are no longer required to contain *only* POD types. So beware of that. — David Rodríguez - dribeas, Apr 13 '11 at 17:04

score 7 · Accepted Answer · answered Apr 13 '11 at 16:13

7

Simply:

#include <algorithm>
std::swap(u1, u2);

Unions are both copy-constructible and assignable, so there is no problem.

answered Apr 13 '11 at 16:13

Jan Hudec

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I originally had that, but I think vc++2008 barfed on it. I'll look into it again. – Evan Teran Apr 13 '11 at 16:17
Come to think about it, previously the union was anonymous (it no longer is), perhaps vc++ just didn't like swapping anonymous unions? – Evan Teran Apr 13 '11 at 16:19
3

@Evan Teran: §14.3.1 [temp.arg.type]/2 *A local type, a type with no linkage, an **unnamed type** or a type compounded from any of these types **shall not be** used as **a template-argument** for a template type-parameter.* So basically yes, you cannot use an *unnamed* union as a type argument to a template, and `std::swap` is a template. – David Rodríguez - dribeas Apr 13 '11 at 17:08

score 3 · Answer 2 · answered Apr 13 '11 at 16:07

3

Why not use classical swap solution?

U u1, u2;
// ...
U t = u1;
u1 = u2;
u2 = t;

This is expected to work since assignment of unions is valid.

answered Apr 13 '11 at 16:07

andrew_d

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5

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One should just use `std::swap`, though. – GManNickG Apr 13 '11 at 16:23
@GMan The above will work in pure C as well (probably moving temporary definition to the up), while std::swap is only for C++ – andrew_d Apr 13 '11 at 16:51
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And we care about C in a C++ question because...? – GManNickG Apr 13 '11 at 16:52

proper way to swap unions?

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