Copy Elision: Guaranteed and Merely Likely

Since C++17, returning a prvalue is guaranteed elision: no copy or move exists even conceptually. The object is constructed directly in its final home:

std::string build() { return std::string(1000, 'x'); }   // zero copies, zero moves — by the language
auto s = build();                                        // one allocation, one string

NRVO — the powerful courtesy

Returning a named local (NRVO) is not guaranteed, but mainstream compilers do it for simple flows:

std::string accumulate(int n) {
    std::string out;              // named local: NRVO candidate
    for (int i = 0; i < n; ++i) out += 'a';
    return out;                   // usually constructed directly in the caller's storage
}

The anti-pattern: return std::move(local)

return std::move(local); disables NRVO (the expression is no longer the local's name) and forces a move — strictly worse than return local;, which moves anyway when elision fails. Compilers flag it (-Wpessimizing-move).

Where elision never applies

• Assignment (only initialization elides),
• parameters (they are copies or references from the start),
• returning a member or parameter (only a local's NRVO or a prvalue return elides).

Count constructions with an instrumented type in the practice — the counters are the proof.