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Do they work across different object files? Do they work across different DLLs?
I know this depends on the compiler. I'm curious if there are any compilers and optimization settings that will make this work.
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Copy elision (NRVO) is allowed there and is routinely performed by most compilers, but is still non-guaranteed, and the widget class cannot be non-copyable non-movable.
Copy elision is a compiler optimization technique that eliminates unnecessary copying/moving of objects.
In the context of the C++ programming language, return value optimization (RVO) is a compiler optimization that involves eliminating the temporary object created to hold a function's return value. RVO is allowed to change the observable behaviour of the resulting program by the C++ standard.
> Note also that C doesn't have return-value-optimization, hence all your struct-returning functions will cause a call to memcpy (won't happen when compiled in C++ mode of course). What ? RVO is precisely needed because a copy in C++ can run arbitrary code and so is not as easy to ellide as a memcpy.
Normally, yes, but in principle, using Link-Time-Optimization (-flto for GCC/Clang compilers and linkers) or Link-Time-Code-Generation (/LTCG and /GL for MSVC's compiler and linker), the compiler and linker can leverage their shared knowledge and perhaps inline code and elide copies. GCC's manual states:
[...] this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Note this will not work with DLLs, because a shared library's code is fixed and already fully compiled.
RVO only needs information about the function itself (as it constructs the function's return value in-place instead of copying/moving on return. This will likely work without the aboce options.
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