cpp core · medium

RVO & Copy Elision

Return-value optimization builds the result directly in the caller's slot, eliminating the copy/move; std::move on a return can disable NRVO.

The code

struct Widget { /* expensive to copy */
};

Widget make() {
    Widget w; // a local
    return w; // NRVO candidate
}
Widget obj = make(); // copy elision: obj built in place

// pitfall:
return std::move(w); // pessimization - can disable NRVO

What this lesson walks through

1. 01Returning a big object by value
2. 02Naive: build a local in make()
3. 03...then move it out (+1)
4. 04RVO: build directly in obj
5. 05Mandatory for prvalues (C++17)
6. 06NRVO for named locals
7. 07Pitfall: don't std::move the return

make() returns a Widget by value, and main stores it in obj. Naively that sounds like: build a Widget, then copy it into obj. Does it really copy?