Fun with C++26 reflection: Keyword Arguments

Re:Fun with C++26 reflection: Keyword Arguments by drysine on Tuesday February 11, 2025 @08:26AM (#43012474)

In a general case you can't do that with separate compilation. [0]

  struct A { A(A*); };

  A* f(struct B *b);

  struct B {
    A a1;
    A a2;
    B(): a1(f(this)), a2(f(this)) {}
  };

  //in a different translation unit
  A* f(B *b)
  {
    return &b->a1; //or a2, we don't know
  }

[0] https://godbolt.org/z/xMb64ssYK

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Re:Fun with C++26 reflection: Keyword Arguments by kccqzy on Tuesday February 11, 2025 @10:45AM (#43014158)

Your code snippet does not use the designated initializer feature that this comment thread is talking about.

Furthermore your code possibly contains undefined behavior depending on the behavior of the constructor of A.

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Re:Fun with C++26 reflection: Keyword Arguments by drysine on Monday February 17, 2025 @03:33AM (#43076614)

As I stated above, it's the same kind of situation. See my another comment in this thread for an example with designated initializers.

>your code possibly contains undefined behavior

Only if f() returns a pointer to a2 (which is my point). Or did you imply that in the case when f() returns a pointer to a1 and it gets passed to the constructor of a1, provenance matters?

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Re:Fun with C++26 reflection: Keyword Arguments by drysine on Thursday February 20, 2025 @03:34PM (#43119808)

Actually, provenance matters:

"During the construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified." [0]

Reading an unspecified value isn't UB, that's good, but I don't understand why the standard says 'unspecified' because it clearly can be indeterminate if a sub-object hasn't been initialized yet.

[0] https://eel.is/c++draft/class.cdtor#2

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