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### 2055. `std::move` in `std::accumulate` and other algorithms

Section: 27.10 [numeric.ops] Status: Resolved Submitter: Chris Jefferson Opened: 2011-01-01 Last modified: 2020-09-06

Priority: 3

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Discussion:

The C++0x draft says `std::accumulate` uses: `acc = binary_op(acc, *i)`.

Eelis van der Weegen has pointed out, on the libstdc++ mailing list, that using `acc = binary_op(std::move(acc), *i)` can lead to massive improvements (particularly, it means accumulating strings is linear rather than quadratic).

Consider the simple case, accumulating a bunch of strings of length 1 (the same argument holds for other length buffers). For strings `s` and `t`, `s+t` takes time `length(s)+length(t)`, as you have to copy both `s` and `t` into a new buffer.

So in accumulating `n` strings, step `i` adds a string of length `i-1` to a string of length 1, so takes time `i`.

Therefore the total time taken is: `1+2+3+...+n` = O(`n2`)

`std::move(s)+t`, for a "good" implementation, is amortized time `length(t)`, like `vector`, just copy `t` onto the end of the buffer. So the total time taken is:

`1+1+1+...+1` (`n` times) = O(`n`). This is the same as `push_back` on a `vector`.

I'm trying to decide if this implementation might already be allowed. I suspect it might not be (although I can't imagine any sensible code it would break). There are other algorithms which could benefit similarly (`inner_product`, `partial_sum` and `adjacent_difference` are the most obvious).

Is there any general wording for "you can use rvalues of temporaries"?

The reflector discussion starting with message c++std-lib-29763 came to the conclusion that above example is not covered by the "as-if" rules and that enabling this behaviour would seem quite useful.

[ 2011 Bloomington ]

Moved to NAD Future. This would be a larger change than we would consider for a simple TC.

[2017-02 in Kona, LEWG responds]

Like the goal.

What is broken by adding std::move() on the non-binary-op version?

A different overload might be selected, and that would be a breakage. Is it breakage that we should care about?

We need to encourage value semantics.

Need a paper. What guidance do we give?

Use std::reduce() (uses generalized sum) instead of accumulate which doesn’t suffer it.

Inner_product and adjacent_difference also. adjacent_difference solves it half-way for “val” object, but misses the opportunity for passing acc as `std::move(acc)`.

[2017-06-02 Issues Telecon]

Ville to encourage Eelis to write a paper on the algorithms in <numeric>, not just for accumulate.

Howard pointed out that this has already been done for the algorithms in <algorithm>

Status to Open; Priority 3

[2017-11-11, Albuquerque]

This was resolved by the adoption of P0616r0.

Proposed resolution: