Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The purpose of the value_type of an iterator is to define the type that is returned when that iterator is dereferenced. For output iterators, the only legitimate use of the dereference operator is when it is used in conjunction with the assignment operator--in the form of <code>*output_iterator = value</code>. The type that is returned when dereferencing an output iterator does not necessarily have any direct relationship with the types that can be stored via the output iterator. The only needed relationship is that there be some way of assigning the latter types to the former type.</p> <p>Moreover, the output iterator can store values of multiple types, and these types do not have to have any relationship with one another. Take for example the <code>null_output_iterator</code> described in <a href="https://stackoverflow.com/questions/335930/335972#335972">Discarding the output of a function that needs an output iterator</a>. That iterator can accept for storage any type of value.</p>
    singulars
    1. This table or related slice is empty.
    1. POOutput iterator's value_type
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USCommunity
    1. USMatthew T. Staebler
    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. COWhen you say _"The purpose of the value_type of an iterator is to define the type that is returned when that iterator is dereferenced"_, can you please specify the reference for that? A wording in the standard, perhaps? In section 24.3.1 of the standard I see no details about what `value_type` is for, only that output iterators may set it to void. In the TCPL3e, section 19.2.2, Stroustrup says only that "`value_type` is the type of element".
      singulars
      1. PO
      1. USwilhelmtell
    2. COFrom section 24.1 of the standard, "All iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator." From section 24.3.1, "[...] it is required that if Iterator is the type of an iterator, [...] iterator_traits<Iterator>::value_type be defined as the iterator's [...] value type [...] ." Also from section 24.3.1, "In the case of an output iterator, [...] iterator_traits<Iterator>::value_type [... is ...] defined as void."
      singulars
      1. PO
      1. USMatthew T. Staebler
    3. COOk, I know that output iterators have `value_type` set to `void`. With `*i` being the `value_type` of `i`, and the programmer not allowed to dereference `i`, there's still a leap into allowing `*i` to be `void`. It needn't be `void`. There are other, valid uses for `value_type`. I'm not debating this, I just don't understand the leap from "you can't dereference an output iterator" to "let's define the `value_type` of an output iterator to `void`".
      singulars
      1. PO
      1. USwilhelmtell
 

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