Why Do Standard Iterators Use Half-Open Ranges ([begin, end))?

Half-Open Range Intervals in Standard Iterators: Rationale

The Standard Library defines the end iterator of a range to be one element past the last element in the sequence, rather than pointing exactly at the last element. This design choice raises the question of why this convention was adopted.

Dijkstra's Key Arguments

According to Edsger W. Dijkstra, who had a significant influence on the Standard, there are two primary reasons for the [begin, end) interval:

1. Simplified Size Calculation: The size of a range can be easily determined as the difference between end and begin.
2. Natural Empty Sequence Representation: The convention naturally represents empty sequences, avoiding the need for a "one-before-the-beginning" special value.

Advantages in Complex Iterations

The half-open range presents advantages when dealing with nested or iterated calls to range-based constructions. By contrast, using doubly-closed ranges would result in off-by-one errors and more complex code.

Justification for Zero-Based Counting

The Standard also favors zero-based counting within these ranges. With the [begin, end) convention, a range of N elements naturally translates to [0, N), eliminating the need for adjustments.

Conclusion

The [begin, end) convention in Standard iterators is a result of thoughtful design considerations that enhance simplicity, readability, and efficiency in range-based algorithms. By avoiding the use of inclusive end iterators, the Standard Library simplifies the manipulation of sequences, particularly in complex scenarios involving multiple nested ranges.

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