Can regex_iterators be used on a custom string type?

Question

I have only found examples of regex_iterators being intialised as

regex_iterator::<string::iterator>

If I have a class which contains a sequence of characters, such as:

class fooString {
private:
  deque<bar> data;

  // ... other functionality ...
}

Where bar is a class or struct that is compatible with std::to_string

How would I make fooString and bar compatible with regex so that I could call

fooString myFoo = ...
regex e( /* some regex notation */);
regex_iterator::<fooString::iterator> regIt(myFoo.begin(), myFoo.end(), e);

...and be able to iterate over it as I would with a string?

I tried the following experiement:

    std::deque<int> testDeque { 4, 5, 5, 5, 6, 7 };
    std::regex e("5{1,3}");
    std::regex_iterator<std::deque<int>::iterator> 
    regIt(testDeque.begin(), testDeque.end(), e);

And get the compilation error

1>C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\regex(2271): error C2027: use of undefined type 'std::regex_traits<_Elem>'
with  
[  
   _Elem=int  
]

So I don't think the bidirectional iterator is enough since deque complies with that, I think the 'bar' object needs to be compatible. But I'm really not sure.

According to this std::regex_iterator reference, the iterator needs to be a bidirectional iterator. So if your iterator fulfills the requirements of a bidirectional iterator, you're good to go. — Some programmer dude
– Some programmer dude, Commented Oct 12, 2017 at 12:56
@Someprogrammerdude I think the data type Bar also needs to fullfill some particular property, but I don't know what it is. — Ian
– Ian, Commented Oct 12, 2017 at 15:02

AndyG · Accepted Answer · 2017-10-12 15:03:19Z

2

fooString must satisfy the requirements of a bidirectional iterator. [bidirectional.iterators] cppreference's documentation is extremely close to the standard wording, so you'd be okay following it.

I will add the relevant portions here:

The iterator, r, must satisfy both operations --r and r++
- these operations return a reference to an iterator of the same type
*r must return a reference to the type the iterator is iterating over.
- a reference type according to what cppreference lists for iterator_traits

If your BidirectionalIterator::value_type is not a standard character type (anything you could make a std::basic_string with, e.g., char), you'll also need to specialize regex_traits, although this is going down quite the rabbit hole.

edited Oct 12, 2017 at 15:03

answered Oct 12, 2017 at 13:29

AndyG

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5 Comments

Ian Over a year ago

I tried creating a simple test using regex_iterator<std::deque<uint32_t>::iterator> however the constructor complains that "regex(2271): error C2027: use of undefined type 'std::regex_traits<_Elem>' - with _Elem=unsigned int So unless I'm mistaken satifying the bidirectional iterator isn't enough - regex only works with some data types.

AndyG Over a year ago

@Ian: Why uint32_t? You'll need a character type.

Ian Over a year ago

for my real situation I'm using a custom struct (Bar), I picked uint32_t because I thought it was a simple type to test with. I also tested with int with the same result. - What I'm really asking is how do I make a FooString containing a structure of Bars work? I think both the FooString and the Bar need some special sauce and while FooString might be the bidirectional iterator I don't know what it is for Bar.

AndyG Over a year ago

@Ian: I have updated the post to discuss regex_traits, but I don't understand why you changed to int instead of a char it works fine with char

Ian Over a year ago

that regex_traits is exactly what I needed. Implementing custom behavior for types other than char and w_char will achieve what I need! Thanks.

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