this post was submitted on 17 Mar 2024
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How can I force a copy of a C++ value? - The Old New Thing (devblogs.microsoft.com)
submitted 8 months ago by lysdexic to c/cpp
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[–] [email protected] 2 points 8 months ago (1 children)

The fact that the solution is not to simply invoke the copy constructor, but to (portably!) write an ungodly mess that uses double-references, full noexcept(noexcept(repeat_of_body)) daisy-chaining, std::forward (a function that doesn't forward), and probably constexpr, and probably explicit(explicit(...)) daisy-chaining, and probably requires(requires(...)) daisy-chaining, and probably constinit, and probably structured bindings, and probably yet another form of auto, and so on..., demonstrates the problem with C++.

There's a place for macros, and this example shows it is precisely it. I'd just write MAKE_COPY(x).

[–] [email protected] 3 points 8 months ago* (last edited 8 months ago) (1 children)

If you look at it... The answer is actually to invoke the copy constructor. They're just complaining that, that can be unwieldy, then going on to say ... "actually if you're using C++17 this is all really easy just say auto(x) instead of writing your own copy constructor!"

It's a lot of words (IMO) without that much substance.

The macro would be unnecessary complexity vs their pre-C++17 compatible copy function FWIW.

The other alternative here which is why I think this is kind of ridiculous is 9/10 you can just change the function you're calling to pass a copy rather than passing a reference if that's what you really want.

[–] [email protected] 1 points 8 months ago

The other alternative here which is why I think this is kind of ridiculous is 9/10 you can just change the function you’re calling to pass a copy rather than passing a reference if that’s what you really want.

...That's a pretty good point. Use the type system to work for you. If you want a value thing, just get it by-value. No need for macros, no need for wildcard ampersands, no need for constexpr constinit consteval static inline noexcept(noexcept(...)) requires(requires(...)) explicit(explicit(...)) { body } -> decltype(body).