4 ms·

Type punning in unions is best done using memcpy. Compilers know what it does and optimize it: http://blog.regehr.org/archives/959 http://blog.regehr.org/archiv

by Kristine1975 11y ago

Type punning in unions is best done using memcpy. Compilers know what it does and optimize it: http://blog.regehr.org/archives/959 http://blog.regehr.org/archives/959 As for integer overflow at least gcc and clang have built-in functions that generate optimal code by checking the CPU's overflow flag: http://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins http://clang.llvm.org/docs/LanguageExtensions.html#checked-a... and https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins...

colin_mccabe 11y ago
C99 Techincal Corrigendum 3 clarifies that union-based type punning is valid by amending 6.5.2.3 with this footnote: If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation. (copied from a comment on blog.regehr.org) This is actually a really common area of misunderstanding. But ultimately, one of the main uses of unions is to provide multiple ways to access the same data.
- physguy1123 11y ago
  In C, yes, type-punning with union is valid. However, in C++, it is considered undefined behavior and may get removed by the compiler if it sees fit.
  - colin_mccabe 11y ago
    Thanks for the clarification. Do the new C++ standards explicitly discuss this anywhere?
    - physguy1123 11y ago
      There's this stackoverflow qanswer: http://stackoverflow.com/questions/11373203/accessing-inactive-union-member-undefined-behavior http://stackoverflow.com/questions/11373203/accessing-inacti... where answers try to distill the standard into something legible by humans. Unsurprisingly, they come to different conclusions. The overall interpretation in some situations, if you look at things just right, you can do type punning in C++. But it's pretty risky since you're a LOC away from the compiler deleting your function. Or, you can't really do it at all safely. Which I would conclude from the above anyways.

colin_mccabe 11y ago

C99 Techincal Corrigendum 3 clarifies that union-based type punning is valid by amending 6.5.2.3 with this footnote: If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation. (copied from a comment on blog.regehr.org) This is actually a really common area of misunderstanding. But ultimately, one of the main uses of unions is to provide multiple ways to access the same data.

physguy1123 11y ago
In C, yes, type-punning with union is valid. However, in C++, it is considered undefined behavior and may get removed by the compiler if it sees fit.
- colin_mccabe 11y ago
  Thanks for the clarification. Do the new C++ standards explicitly discuss this anywhere?
  - physguy1123 11y ago
    There's this stackoverflow qanswer: http://stackoverflow.com/questions/11373203/accessing-inactive-union-member-undefined-behavior http://stackoverflow.com/questions/11373203/accessing-inacti... where answers try to distill the standard into something legible by humans. Unsurprisingly, they come to different conclusions. The overall interpretation in some situations, if you look at things just right, you can do type punning in C++. But it's pretty risky since you're a LOC away from the compiler deleting your function. Or, you can't really do it at all safely. Which I would conclude from the above anyways.