Imagine no_rvo is defined in a different file than main so that when compiling main the compiler will only see the declaration X no_rvo(X x); and will have no idea whether the object of type X returned has any relation to the argument. From what it knows at that point, the implementation of no_rvo could … Read more
According to Richard Smith: This is a defect in the standard wording. Copy elision cannot be guaranteed when initializing a base class subobject, because base classes can have different layout than the corresponding complete object type.
Yes, by all means, you should write fn initialize() -> Vec<Vec<MyStruct>> { … } (By the way, a Vec is not that large – it’s only 3 pointer-sized integers) Rust has RVO, and this is advertised in guides. You can see it yourself with this code: #[inline(never)] fn initialize() -> Vec<i32> { Vec::new() } fn … Read more
The compiler may NRVO into a temp space, or move construct into a temp space. From there it will move assign x. Update: Any time you’re tempted to optimize with rvalue references, and you’re not positive of the results, create yourself an example class that keeps track of its state: constructed default constructed moved from … Read more
Dave Abrahams has a pretty comprehensive analysis of the speed of passing/returning values. Short answer, if you need to return a value then return a value. Don’t use output references because the compiler does it anyway. Of course there are caveats, so you should read that article.
Here’s why copy elision doesn’t make sense for parameters. It’s really about the implementation of the concept at the compiler level. Copy elision works by essentially constructing the return value in-place. The value isn’t copied out; it’s created directly in its intended destination. It’s the caller who provides the space for the intended output, and … Read more
In two first cases RVO optimization will take place. RVO is old feature and most compilers supports it. The last case is so called NRVO (Named RVO). That’s relatively new feature of C++. Standard allows, but doesn’t require implementation of NRVO (as well as RVO), but some compilers supports it. You could read more about … Read more
Does returning a local variable return a copy and destroy the original? The final answer to your question is that it depends on whether or not optimization is enabled. So lets discuss each case separately. Note also that since the given output in the original question is for C++17, the below discussion is also for … Read more
The cases where copy and move elision is allowed is found in section 12.8 ยง31 of the Standard (version N3690): When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side … Read more
Use exclusively the first method: Foo f() { Foo result; mangle(result); return result; } This will already allow the use of the move constructor, if one is available. In fact, a local variable can bind to an rvalue reference in a return statement precisely when copy elision is allowed. Your second version actively prohibits copy … Read more