Why does std::find_if used on std::istream_iterators seem to return the last element?

Question

I am learning C++ through Accelerated C++ by Andrew Koenig and Barbara E. Moo. I am trying to understand how input operators work and their relation to STL algorithms.

Here is the piece of code that confuses me:

#include <iostream>
#include <iterator>
#include <algorithm>
#include <cctype>

bool space(char c)
{
    return std::isspace(c);
}

int main()
{
    std::istream_iterator<char> i = std::find_if(
        std::istream_iterator<char>(std::cin),
        std::istream_iterator<char>(),
        space);
    std::cout << *i << std::endl;
    return 0;
}

The code compiles fine but when I run it on any input what it outputs is the last character entered. For example, I expected the output of 123 456 to be but it actually is 6. Similarly, I expected the output of 123456 to be some error because we try to access an element that doesn't exist, yet the output is again . What am I missing?

Answer 1

Why doesn't 123 456 as input produce as output?

From cppreference :

When reading characters, std::istream_iterator skips whitespace by default (unless disabled with std::noskipws or equivalent), while std::istreambuf_iterator does not.

Switching to std::istreambuf_iterator gives the desired behaviour: https://wandbox.org/permlink/RRt8kvyqyvbO1p8m

---

Why doesn't 123456 as input produce an error?

If there is no space in the input, find_if will return its 2nd argument, which is the end-of-stream iterator.

Dereferencing the end-of-stream iterator is undefined behaviour , which is not guaranteed to produce an error. In fact it doesn't guarantee anything, which means your program might just print the last character in your input.