Elusive Copy

What this C++ language semantics question tests

This is a hard C++ language-knowledge question that probes deep familiarity with how the standard specifies program behaviour—particularly the subtleties introduced or clarified in C++17 and later revisions. It rewards candidates who read the standard closely and understand the difference between what code *looks* like it does and what it *actually* does according to the language rules.

Questions of this type are common in interviews at firms that rely on correctness-critical or high-performance C++ codebases. They test whether you can reason about obscure but real pitfalls: undefined behaviour, unspecified ordering, side effects, and the precise semantics of language constructs that appear simple on the surface. A strong answer requires you to trace through the program's execution according to the C++17 standard, not just intuition.

• Order of evaluation and sequence points
• Undefined and unspecified behaviour
• Standard library guarantees and contracts
• Compiler optimisation and observable behaviour