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>at the very least C should make reading from a different union member than was last written to "implementation defined" That's how it is :) The standard even

by pubby 8y ago

>at the very least C should make reading from a different union member than was last written to "implementation defined" That's how it is :) The standard even describes union behavior as "type punning". I don't know if C++ allows it though.

maccard 8y ago
Don’t think c++ allows it. Last time I checked, if you use any modern compiler, and do a memcpy, and check the assembly, it does the right thing.
- saagarjha 8y ago
  Yes, that's the "approved" way of doing type punning. Any decent compiler will immediately optimize that down to nothing.
simias 8y ago
You're right, apparently it changed in C99 so my knowledge is pretty out of date. Thank you for pointing it out.
TorKlingberg 8y ago
Type punning through a union is allowed, buy you have to be careful so the compiler can see the union. union { int i; float f; } u; You can't safely call a function with pointers to the members like func(&u.i, &u.f), because when func was compiled, the compiler couldn't know that the arguments are pointers to union members. This sort of thing scares me, because what the compiler can "see" isn't well defined. Is it ok if func is in the same file? Probably not. How about if I don't send the pointers to an other function and just use them locally? Probably ok, but I wouldn't trust it. If you want to type pun with a union, make sure to always access the members through the union, not with direct pointers. The only method of type punning I 100% trust is memcpy, and if you get lucky the compiler might optimize away the overhead for you.
- boomlinde 8y ago
  Assuming we're talking C, not C++, the compiler doesn't need to know that the arguments are pointers to union members, because the union members are all aligned with the union itself and have the same address. So &u.f is not some magic special pointer, it's the address of a float. So is (float *)&u. The example in the standard I think you're referring to relates to a special guarantee made about unions of structs that have a common initial sequence, that their common initial sequence can be read from either union member (AFAIK a guarantee not otherwise made about structs that share a common initial sequence). If your func(&u.i &u.f) example fails it may be because the standard is picky about pointers of different types to the same memory. I have no idea whether it is, because I've never had to do anything like your example.
  - BruiseLee 8y ago
    The func can be implemented like this: float func(int *i, float *f) { *i = 42; return *f; } Since the compiler assumes that the pointers i and f do not alias, it has a right to generate the code that is equivalent to: float func(int *i, float *f) { float result = *f; *i = 42; return result; } But this will change the result of the function. So in practice if you want to do type punning using unions, the compiler must know about it.
    - boomlinde 8y ago
      Sure, by not accessing the members through the union type you lose the information the compiler needs to relax strict aliasing assumptions for union access. It's a potential footgun, but IMO only with careful aiming. It's also not strictly related to union member pointers but applies generally to aliased pointers.
  - vyodaiken 8y ago
    The compiler and standard are both full of contradictory, useless, and counter-productive rules on aliasing. But both gcc and clang now support _may_alias_ and -fno-strict-aliasing without which they would have made the language completely unusable.

by pubby 8y ago

maccard 8y ago

Don’t think c++ allows it. Last time I checked, if you use any modern compiler, and do a memcpy, and check the assembly, it does the right thing.

saagarjha 8y ago
Yes, that's the "approved" way of doing type punning. Any decent compiler will immediately optimize that down to nothing.

simias 8y ago

You're right, apparently it changed in C99 so my knowledge is pretty out of date. Thank you for pointing it out.

TorKlingberg 8y ago

Type punning through a union is allowed, buy you have to be careful so the compiler can see the union. union { int i; float f; } u; You can't safely call a function with pointers to the members like func(&u.i, &u.f), because when func was compiled, the compiler couldn't know that the arguments are pointers to union members. This sort of thing scares me, because what the compiler can "see" isn't well defined. Is it ok if func is in the same file? Probably not. How about if I don't send the pointers to an other function and just use them locally? Probably ok, but I wouldn't trust it. If you want to type pun with a union, make sure to always access the members through the union, not with direct pointers. The only method of type punning I 100% trust is memcpy, and if you get lucky the compiler might optimize away the overhead for you.

boomlinde 8y ago
Assuming we're talking C, not C++, the compiler doesn't need to know that the arguments are pointers to union members, because the union members are all aligned with the union itself and have the same address. So &u.f is not some magic special pointer, it's the address of a float. So is (float *)&u. The example in the standard I think you're referring to relates to a special guarantee made about unions of structs that have a common initial sequence, that their common initial sequence can be read from either union member (AFAIK a guarantee not otherwise made about structs that share a common initial sequence). If your func(&u.i &u.f) example fails it may be because the standard is picky about pointers of different types to the same memory. I have no idea whether it is, because I've never had to do anything like your example.
- BruiseLee 8y ago
  The func can be implemented like this: float func(int *i, float *f) { *i = 42; return *f; } Since the compiler assumes that the pointers i and f do not alias, it has a right to generate the code that is equivalent to: float func(int *i, float *f) { float result = *f; *i = 42; return result; } But this will change the result of the function. So in practice if you want to do type punning using unions, the compiler must know about it.
  - boomlinde 8y ago
    Sure, by not accessing the members through the union type you lose the information the compiler needs to relax strict aliasing assumptions for union access. It's a potential footgun, but IMO only with careful aiming. It's also not strictly related to union member pointers but applies generally to aliased pointers.
- vyodaiken 8y ago
  The compiler and standard are both full of contradictory, useless, and counter-productive rules on aliasing. But both gcc and clang now support _may_alias_ and -fno-strict-aliasing without which they would have made the language completely unusable.