What is half open range and off the end value

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A half-open range is one which includes the first element, but excludes the last one.

The range [1,5) is half-open, and consists of the values 1, 2, 3 and 4.

“off the end” or “past the end” refers to the element just after the end of a sequence, and is special in that iterators are allowed to point to it (but you may not look at the actual value, because it doesn’t exist)

For example, in the following code:

char arr[] = {'a', 'b', 'c', 'd'};

char* first = arr
char* last = arr + 4;

first now points to the first element of the array, while last points one past the end of the array. We are allowed to point one past the end of the array (but not two past), but we’re not allowed to try to access the element at that position:

// legal, because first points to a member of the array
char firstChar = *first;
// illegal because last points *past* the end of the array
char lastChar = *last;

Our two pointers, first and last together define a range, of all the elements between them.

If it is a half open range, then it contains the element pointed to by first, and all the elements in between, but not the element pointed to by last (which is good, because it doesn’t actually point to a valid element)

In C++, all the standard library algorithms operate on such half open ranges. For example, if I want to copy the entire array to some other location dest, I do this:

std::copy(first, last, dest)

A simple for-loop typically follows a similar pattern:

for (int i = 0; i < 4; ++i) {
    // do something with arr[i]
}

This loop goes from 0 to 4, but it excludes the end value, so the range of indices covered is half-open, specifically [0, 4)

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