What's with allocator in vector?

Question

vector<int> data(istream_iterator<int>(cin),
istream_iterator<int>{});   cout<<"Size is : " << data.size() << endl; //compile success

vector<int> data1(istream_iterator<int>(cin),
std::allocator<int>{});   cout<<"Size is : " << data1.size() << endl; //compile failure

 error: no matching function for call to ‘std::vector<int>::vector(std::istream_iterator<int>, std::allocator<int>)’    vector<int> data1(istream_iterator<int>(cin), std::allocator<int>{});

Why is the first statement fine but second? Doesn't the vector take int type allocator in this case? I am experimenting with allocators.

Because the second one doesn't make sense? What does it mean to create a vector from a beginning iterator and an allocator? — NathanOliver
– NathanOliver, Commented Oct 28, 2020 at 13:55
istream_iterator<int>{} represents the matching end iterator for istream_iterator<int>(cin) as begin iterator. (For input iterators, it's valid to use a default constructed (aka. singular iterator) as end iterator - as I recently learnt.) — Scheff's Cat
– Scheff's Cat, Commented Oct 28, 2020 at 13:56
@InQusitive istream_iterator<int>{} is not an allocator. It's a sentinel iterator (end iterator for a stream) — NathanOliver
– NathanOliver, Commented Oct 28, 2020 at 13:57
If you remove the end iterator from the first statement, it tries to use one of the numerous other (but non-matching) constructors. You need the constructor for a range given by begin and end iterator and none of them is provided with a default argument (as this wouldn't make much sense). So, you always have to give both. — Scheff's Cat
– Scheff's Cat, Commented Oct 28, 2020 at 15:04

eerorika · Accepted Answer · 2020-10-28 14:04:05Z

5

Why is the first statement fine

Because vector has a constructor that accepts two iterators (and an allocator with default argument). Those iterators represent beginning and end of an input range.

but second [is not]?

Because vector doesn't have a constructor that accepts a single iterator and an allocator.

answered Oct 28, 2020 at 14:04

eerorika

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2 Comments

Inquisitive Over a year ago

[Not referring my code] Why doesn't vector have single iterator with allocator but is having two iterator with allocator?

eerorika Over a year ago

@InQusitive Because a single iterator cannot represent a range. How could the constructor know how many elements to read from the iterator?

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