Placement NWBI<- FAQ (New/Box/In/Left Arrow)

[vadimcn]

This point is not about space overhead.

I disagree. Elimination of temporaries is the whole point of the placement operator. Otherwise, you might as well write it as Box::new(try!(make_value()))

[eddyb]

Box::new

Box::new was never supposed to exist.

Sadly, it might be a bit late for universal placement, but the goal was to have a single way to allocate/emplace, with maximal efficiency and minimal syntactical overhead.

If successful, Box::new, Vec::push etc. will be deprecated.

[yigal100]

Vec::push cannot be deprecated because there are two possible semantics here - push to the vector and fail if there is no more capacity, or push and reallocate the entire vector if necessary. Unless the API will dictate to replace the latter semantics, arguably the common case atm, with a capacity check before pushing to the vector. Seems to me that such a choice will make for a bad API experience. The former is really an “emplace” as it’s called in C++ land, or better yet, “try_push”.

[rkjnsn]

In C++ land, emplace_back most certainly will reallocate the vector if it is currently at capacity.

[cuviper]

Even plain emplace will reallocate as needed. It’s just insert with running the constructor in-place.

[yigal100]

Ok, I’m confused… Why than does C++ need to have both push_back() and emplace_back()? What’s the difference?

In any case, shouldn’t there be at least some method to indicate where to push/emplace to, e.g. push_front vs. push_back?

[Amanieu]

Ok, I’m confused… Why than does C++ need to have both push_back() and emplace_back()? What’s the difference?

This avoids calling an object’s copy constructor or move constructor by simply constructing it in place. Since Rust doesn’t have move constructors this isn’t as much of an issue.

[pnkfelix]

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