https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106666
Bug ID: 106666
Summary: Anonymous struct incorrectly allows types with
constructors if placed in a known sized array
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: twmouton at gmail dot com
Target Milestone: ---
Consider the following:
#include <iostream>
#include <memory>
struct ConstructMe {
ConstructMe() {
std::cout << "I have been constructed" << std::endl;
}
~ConstructMe() {
std::cout << "I have been destructed" << std::endl;
}
};
struct Foo {
Foo()
: me{ std::make_unique< ConstructMe >() } {
std::cout << "Foo" << std::endl;
}
struct {
std::unique_ptr< ConstructMe > me[1];
};
};
int main() {
Foo f;
return 0;
}
Compiled with g++ 12.1 using only -std=c++17.
If you remove the "[1]" after "me" the compiler will emit the error "error:
member 'std::unique_ptr<ConstructMe> Foo::<unnamed struct>::me' with
constructor not allowed in anonymous aggregate" with similar errors for the
existence of a destructor and copy-assignment operator.
To make matters worse, if you do make this an array then the object is only
constructed if it's included in the member initialization list (in above -
remove the unique_ptr and use just a plain ConstructMe to see that it is not
constructed) and it is never destructed. In the example above using unique_ptr,
this means the memory is leaked when Foo falls out of scope.
Since this is an extension, I am not sure what the correct behavior should be
but it should be consistent regardless of an object is in an array or single
member. I see two options for this.
1. Make it so the objects in an array that have
constructor/destructor/copy-assignment defined emit the same errors as not
having an array.
2. Objects with zero argument constructors should be allowed in an anonymous
struct, and should be constructed/destructed as if they were direct members of
the enclosing struct.
For what it's worth, clang seems to have chosen option 2.
