cpp17 · advanced

C++17: Guaranteed Copy Elision

C++17 reframes prvalues and makes their copy elision mandatory rather than a permitted optimization: a prvalue is now a 'recipe' that materializes into an object only where one is needed, so return T(); and T x = make(); construct the object directly in the destination with no copy or move — even conceptually. Because no copy or move is required, a type may have both its copy and move constructors deleted and still be returned by value from a factory, which enables zero-overhead factories for immovable types (scope guards, mutex-like wrappers, types holding references). The crucial distinction for interviews: this guarantee applies only to prvalues. Named Return Value Optimization — returning a named local with return obj; — remains an optional optimization that still requires an accessible move or copy constructor as a fallback, so it is wrong to claim every return is guaranteed to elide.

The code

struct Big {
    Big();
    Big(const Big&) = delete;
    Big(Big&&) = delete;
};

Big make() {
    return Big();
} // OK in C++17! no copy/move needed
Big b = make(); // OK: constructed directly into b

// A prvalue is now 'a recipe to construct', materialized only when needed
// Pre-C++17 this required an accessible copy/move ctor (even if elided).

What this lesson walks through

1. 01Prvalues no longer copy
2. 02What it enables
3. 03Gotcha — only prvalues are guaranteed
4. 04Elision vs NRVO — know the line

In C++17, copy elision for prvalues is mandatory, not an optimization. Returning a temporary (return Big();) and initializing from one (Big b = make();) construct the object directly in the destination — no copy or move ever happens, even conceptually. This is the new prvalue model: a prvalue is a 'recipe' that materializes into an object only where one is needed.