Copy elision vs rvo

Question

What is difference between copy elision and return value optimization?

Answer 1

Ketan Lalcheta Return Value Optimization is a subset of Copy Elision.

Both are compiler optimizations designed to avoid resource/time costly copying of Class objects.

Return Value Optimization focuses only on avoiding these copies in regard to functions which return-by-value.
A distinction is made between 2 forms of RVO:
👉 named RVO - variable for Class object is created inside function and later passed as return value
👉 unnamed RVO - Class object created directly in the function's return statement

Copy Elision more broadly covers:
👉 function return-by-value of Class objects
👉 initializing a Class object from another object of same Class
👉 passing Object values in try/throw - catch exception handling

In the C++17 standard, certain categories of copy elision should be guaranteed to always occur,
Especially unnamed return value optimization.

Answer 2

The question should be
When is copy elision used in RVO.

"Copy elision is a technique to skip calling the copy constructor so as not to pay for the overhead."

That's the part that caught my attention from this SO thread:

https://stackoverflow.com/questions/2143787/what-is-copy-elision-and-how-does-it-optimize-the-copy-and-swap-idiom

Answer 3

Bob_Li copy elision is always used in RVO.
Or more correctly, RVO always uses copy elision.

RVO is just the most common example of copy elision. That and the fact a distinction is made between named and unnamed RVO is why RVO has its own term.

Answer 4

Shardis Wolfe
yes, but under what conditions the compiler enforce it have a lot of nuance. Feels like a good subject to explore.