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[class.copy.elision] p2 incorrect comment in copy elision example #4926

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RazvanAM opened this issue Sep 22, 2021 · 4 comments · Fixed by #4928
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[class.copy.elision] p2 incorrect comment in copy elision example #4926

RazvanAM opened this issue Sep 22, 2021 · 4 comments · Fixed by #4928
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@jensmaurer

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@RazvanAM
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Paragraph 2 of [class.copy.elision] (https://eel.is/c++draft/class.copy.elision#2) contains the following example:

[...]

constexpr A g() {
  A loc;
  return loc;
}

constexpr A a;          // well-formed, a.p points to a
constexpr A b = g();    // error: b.p would be dangling ([expr.const])

void h() {
  A c = g();            // well-formed, c.p may point to c or to an ephemeral temporary
}

I believe the comment "well-formed, c.p may point to c or to an ephemeral temporary" is not entirely correct, because there is no temporary in this scenario.

This comment was originally added with the resolution of CWG2002. The example used in that resolution was:

[...]

struct A {
    void *p;
    constexpr A(): p(this) {}
  };

  constexpr A a;        // well-formed, a.p points to a
  constexpr A b = A();  // well-formed, b.p points to b

  void g() {
    A c = A();          // well-formed, c.p may point to c or to an ephemeral temporary
  }

In this resolution the comment "well-formed, c.p may point to c or to an ephemeral temporary" correctly reflects the code.

After the resolution of CWG2002 the code in the example was modified in 95321805a3, but the comment was not changed to reflect this.
@xmh0511
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xmh0511 commented Sep 23, 2021 •
edited

Agree that comment is not correct. Since both [class.copy.elision]p1 and [expr.const] p1 says

Copy elision is not permitted where an expression is evaluated in a context requiring a constant expression ([expr.const]) and in constant initialization ([basic.start.static]).

Hence, for A c = g(); , c.p may point to loc, or point to an ephemeral temporary as per [class.temporary] p3, depends on which initializer expression performs the copy-initialization of c.

@jensmaurer
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There is no temporary here; the result of g() is an rvalue whose target object is c.

@jensmaurer jensmaurer self-assigned this Sep 23, 2021
@jensmaurer jensmaurer mentioned this issue Sep 23, 2021
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@xmh0511
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xmh0511 commented Sep 24, 2021

When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor ([special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object).

There can have a temporary here.

@jensmaurer
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jensmaurer commented Sep 24, 2021 •
edited

Good point. I think the pull request is correct regardless, because c.p will be dangling at the end either way.

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[class.copy.elision] Fix comment in example jensmaurer/draft
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@RazvanAM @xmh0511 @jensmaurer