Why for_each returns a function by moving

Question

I read the documentation for std::for_eachhere http://en.cppreference.com/w/cpp/algorithm/for_each and saw that the return valuestd::move(f)

Why does the standard force an input parameter to return value? Could this be changed by default, since the input parameter is passed by value?

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This leads me to the following example when you compile the following code

Something function(Something something) {
    return something;
} 

• The return statement is moving my system with the highest level of optimization ( -O3), why don't most compilers return this return value? Local values ​​are rejected, but function arguments are not ...

• Does C ++ 17 use elite in this case? I read the sentence ( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0135r0.html ), but I don’t quite understand what cases are suitable for a mandatory elixir.

I tried this on Apple LLVM version 8.0.0 (clang-800.0.42.1)on my Mac and on g++ 5.4on Ubuntu 16.04.

Answer 1

This is due to a late change to the move-semantics rules for C ++ 11. The original move proposal did not automatically move when the arguments to the value-value function appeared in the return statement. However, by the end of the C ++ 11 process, this language feature was added.

for_each " ". . , , ++ 11 .

LWG- 2747 ++ 17.

, , , , ( ), , , , , , - , .

: , ++ 17 . , ++ 11, , for_each .

:

, , ?

N4660, ++ 17, ++ 98/03/11/14... , . . N4659 ( ):

15.8.3 / elision [class.copy.elision]

• , / ,...

  • return , ( , ( 18.3)) ( cv-qualification) , / ,

.