When should a parameter be a value, a reference, or a const reference?

A strong answer

A strong candidate frames When should a parameter be a value, a reference, or a const reference? around the exact memory layout, compiler semantics, and hardware execution model covered in Passing by Value, by Reference and by Const Reference. Compare effects on the caller, machine cost, aliasing, and the rule for literals with reference parameters.

Lesson 5 established that a function receives copies. That has two awkward consequences. First, a function that is supposed to change the caller's variable cannot. Here is the classic example, a swap that does nothing useful:

After swapInts(x, y), the variables x and y are unchanged, because the function swapped two copies. Second, copying is wasteful for big objects. A std::string that holds a long text is copied in full on every call, even if the function only wants to look at it.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Predict whether a call changes the caller's variable for value and reference parameters.
• Mastery of core invariant: Explain what a reference is at the machine level and what passing one costs.
• Mastery of core invariant: Choose by value, reference or const reference for a given parameter.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.