Hacker News Reader

Comment by PartiallyTyped | Hacker News Reader

By accepting a reference argument, you introduce pointer deref. Wouldn't it be better to use

    Widget && temp = complicated_long_function(...);
    consume(temp);

Am i missing something?

View on HN

That won’t work. Copy elisions only happen when they are copy constructed from prvalues, not simply an rvalue.

The pointer deref is unavoidable if you pass a large enough object by value, which is what’s happening here. On x86_64, generally values larger than 16 bytes are passed by reference transparently.

I see, thanks!

Your proposed example code does exactly what GP said, pass as a reference (in your case, an rvalue reference).

Not quite, I suggested changing the signature of “consume” to accept a reference. His example doesn’t do anything to invoke copy elision, since his named rvalue reference will be function cast to a normal reference and then copy-constructed into the stack value passed to “consume”.

https://gcc.godbolt.org/z/qd5qnhPT6

How does any copy-construction happen here?

Reply on news.ycombinator.com