On Fri, 18 Nov 2022, 22:00 Segher Boessenkool, wrote: > On Fri, Nov 18, 2022 at 10:35:20PM +0100, Martin Oberzalek via Gcc-help > wrote: > > Am Freitag, dem 18.11.2022 um 16:05 +0100 schrieb Stefan Ring via Gcc- > > help: > > > On Tue, Nov 15, 2022 at 6:48 AM Yubin Ruan via Gcc-help > > > wrote: > > > > Is there any language facility to help us guarantee that at compile > > > > time > > > > (such as some kind of static_assert() ) so that we can be confident > > > > writing > > > > those one-liner ? > > > With c++ language features this may is a solution: > > Nope. Try with -O0 for example. > > There is no way to guarantee copy elision. It isn't even clear what > *exactly* you want guaranteed, what "copy elision" means *exactly*, what > "guaranteeing copy elision" means, etc. > The standard defines what it means here: https://eel.is/c++draft/class.copy.elision Since C++17 it is guaranteed in certain situations: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0135r0.html There is a proposal to guarantee it in more cases, including the one the OP asked about: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2025r2.html That has not been approved for the standard, but it explains the status quo quite well. > If you write clear and simple (which means: not artificially made > complex) and correct source code, you can trust the compiler will > generate good machine code for you (if you use -O2 or such). If not, > bug reports are welcome! > > A C compiler is not a "portable assembler", you have no direct control > over generated code. This is a good thing: the compiler is much better > at writing fast machine code than users are. > > > Segher >