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
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
-fno-elide-constructors The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. [Source: man gcc]
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
RVO/NRVO are clearly allowed under the “as-if” rule in C. In C++ you can get observable side-effects because you’ve overloaded the constructor, destructor, and/or assignment operator to give those side effects (e.g., print something out when one of those operations happens), but in C you don’t have any ability to overload those operators, and the … 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.
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
Introduction For a technical overview – skip to this answer. For common cases where copy elision occurs – skip to this answer. Copy elision is an optimization implemented by most compilers to prevent extra (potentially expensive) copies in certain situations. It makes returning by value or pass-by-value feasible in practice (restrictions apply). It’s the only … Read more