Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>A union reserves space sufficient to store the largest of its members<sup>1</sup>. So, by definition, the same space is reserved for <code>union { double a; byte b }</code> whether you happen to have stored through the double or the byte.</p> <p>Try not to think of a union as a variant type that can be <em>either</em> a byte or a double or an int depending on which member you happened to write most recently. It is in fact a mechanism for overlaying multiple different data types on the same region of memory, so that region is <em>simultaneously</em> a float and an int and a what-have-you in a sort of quantum superimposition. This is especially useful for <a href="http://en.wikipedia.org/wiki/Type_punning" rel="nofollow">type-punning</a> which is when you need to get around the language's type system. </p> <p><sup>1</sup>ANSI/ISO 9899.</p>
    singulars
    1. This table or related slice is empty.
    1. POIs this memoy-usage optimization technique ever possible in case of unions?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USCrashworks
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COMy understanding is that we should think of unions as a variant type and only the member which has been written the more recently can be read and type punning with union is undefined behavior. (The special rules allowing to inspect the common prefix of structures in an union would be without object if it wasn't the case. I'm pretty sure I've already read better references in the standard than that but I can't find them now.)
      singulars
      1. PO
      1. USAProgrammer
    2. COWell, subverting the type system is always undefined behavior, but it's also sometimes just plain necessary when you are trying to do some platform specific things (like fiddling the sign bits of floats). One of the changes from C90 to C99 was to remove any restriction on accessing one member of a union when the last store was to a different one. Embedded programmers often have to do things that make language purists scream.
      singulars
      1. PO
      1. USCrashworks
    3. COThere is a difference between C++ and C here and I answered in the context of C++. First what is common: in both cases assigning to a member makes the bytes unused by this member undefined (it is very clear in C, 6.2.6.1/7 in N1124) and I think the compiler can assume that what it wrote is still there (but I didn't find a statement as clear). What isn't common, in C reading another member than the last written was implementation defined and now got somewhat defined. As far as I know, in C++ it is undefined (at least I didn't find anything hinting otherwise, I'm interested if you know some).
      singulars
      1. PO
      1. USAProgrammer
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload