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[std-proposals] copy constructor of std::vector

From: Federico Kircheis <federico_at_[hidden]>
Date: Tue, 13 Jun 2023 10:34:46 +0200
Hello to everyone, please consider following snippet

#include <vector>
#include <memory>
struct s{
     std::vector<std::unique_ptr<int>> d;
void bar(){
  s s1;
  // fails, as expected, but inconsistent with is_copy_assignable
  auto s2 = s1;
Doesn't the second static_assert bother anyone?
Vector is always is_copy_assignable, but in fact it is a compile-time 
error (auto s2 = s1) does not compile.
Depending on the compiler, compiler errors are also verbose 
(https://godbolt.org/z/3E49c4oT7) and not necessarily easy to understand 
(immagine if s is part of a class hierarchy or a member of another 
An improvement for the end-user is to delete the copy-constructor of s 
by hand, it does not change what programs are valid, but the error 
message is much more easy to understand.
But it does not fix the underlying issue, and the fact that one cannot 
simply follow the "rule of 0" and let the compiler use the defaulted 
copy and move constructors without writing them down.
Does anyone know if there is already a paper trying to fix this issue, 
or if there was one and was rejected?
I assume the main difficulty is specifying something that also works 
with forward-declared types.

Received on 2023-06-13 08:34:53