Questions tagged [iterator-traits]
42 questions
37
votes
1 answer
What are the typical use cases of an iterator_trait
I am new to C++ so please bear with me. I am trying to understand STL iterator_traits. In the book "The C++ Standard Library" the structure iterator_traits is defined as follows:
template
struct iterator_traits {
typedef typename…
san
- 4,144
- 6
- 32
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34
votes
1 answer
Contiguous iterator detection
C++17 introduced the concept of ContiguousIterator http://en.cppreference.com/w/cpp/iterator.
However it doesn't seem that there are plans to have a contiguous_iterator_tag (in the same way we now have random_access_iterator_tag) reported by…
alfC
- 14,261
- 4
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22
votes
4 answers
Why the value_type/difference_type/pointer/reference of back_insert_iterator/front_insert_iterator/insert_iterator are all void?
In my project, I want to split stream into some given type of values, so I implement a template function as
template
TOutputIter SplitSpace(std::istream& IS, TOutputIter result)
{
TElem elem;
while (IS…
Yun Huang
- 4,256
- 7
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- 36
21
votes
2 answers
const_iterator and constness of const_iterator::value_type
Why in STL
std::iterator_traits::value_type
is the same type as
std::iterator_traits::value_type
Why it is designed like that? Shouldn't the first be const T and the second only T? How are you supposed to take the underlying const…
Sogartar
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- 2
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13
votes
3 answers
Why can't I get value_type from iterator_traits?
I'm doing this:
const int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 };
const auto foo = cbegin(arr);
const typename iterator_traits::value_type bar = 1;
I would have expected bar to have the type int. But instead I'm…
Jonathan Mee
- 37,899
- 23
- 129
- 288
11
votes
4 answers
specializing iterator_traits
I'd like to specialize std::iterator_traits<> for iterators of a container class template that does not have the usual nested typedefs (like value_type, difference_type, etc.) and whose source I shouldn't modify. Basically I'd like to do something…
imre
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11
votes
2 answers
Why do algorithms use iterator_traits::value_type instead of iter::value_type?
In an algorithm, I can determine the value_type directly from the iterator via iter::value_type. Why do algorithms use iterator_traits to do the same?
#include
#include
#include
#include
using namespace…
Hardik
- 149
- 6
11
votes
2 answers
Why does reverse_iterator doubly define its nested types?
It appears that the iterator adaptor reverse_iterator doubly defines most of its nested types. In particular, it inherits publicly from std::iterator which exposes iterator_category, value_type, difference_type, pointer and reference. Except for…
TemplateRex
- 69,038
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10
votes
1 answer
Are Forward-Iterators Output-Iterators?
Are ForwardIterators required to be OutputIterators? My current STL-implementation (VS2012) derives forward_iterator_tag from both input_iterator_tag and output_iterator_tag, but I can't find this requirement in the standard [N3485].
MFH
- 1,664
- 3
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7
votes
1 answer
Canonical way to define forward output iterator
How does one define forward-output-iterators in C++11 in a canonical way?
According to the standard a forward_iterator is only a input_iterator. So the corresponding forward_iterator_tag only extends input_iterator_tag. If we are using std::iterator…
MFH
- 1,664
- 3
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5
votes
3 answers
Is There an Example of an Iterator Which Wouldn't use ptrdiff_t as its difference_type?
I see that iterator_traits always defines a difference_type: https://en.cppreference.com/w/cpp/iterator/iterator_traits#Member_types
I'm just wondering why, wouldn't that be ptrdiff_t for every type? Is there an example of an iterator which doesn't…
Jonathan Mee
- 37,899
- 23
- 129
- 288
4
votes
1 answer
Template parameters not used in partial specialization
I have the following code:
template >
class Carray {
// ...
typedef T* pointer;
typedef pointer iterator;
// ...
};
Now I'm trying to do partial specialization for iterator_traits.…
orlp
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4
votes
1 answer
How to avoid specializing iterator_traits for each possible instantiation of a templated iterator?
I want to pimp up my ranged based for loops, for example by enabling reversed iteraton. I managed to get it working to some degree by writing an adaptor, but I am lost on how to make the adpaptor composable.
#include
template
463035818_is_not_an_ai
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4
votes
1 answer
std::iterator_traits divergence between libstdc++ and libc++
Given:
struct Iter {
using value_type = int;
using difference_type = int;
using reference = int;
using pointer = int;
using iterator_category = int;
};
The following works fine with libstc++, but fails to compile against libc++…
jotik
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4
votes
2 answers
Can one make move_iterator from istream_iterator?
Consider following code:
typedef istream_iterator char_itr ;
char_itr eos;
string ll("some text here");
istringstream line_in(ll);
char_itr start(line_in);
move_iterator mstart(start); // !!!
move_iterator…
Piotr G
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