https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82873
Bug ID: 82873
Summary: Generated copy constructor calls constructors for
0-sized array members
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: matthew.fernandez at gmail dot com
Target Milestone: ---
Hello there, I'm trying to do something a little unorthodox and ran into an
undefined reference during linking. It appears that a generated copy
constructor calls another copy constructor that itself is not generated. A
minimised example is:
#include <vector>
class Foo {
std::vector<int> x[0];
};
int main(void) {
Foo a;
Foo b(a);
return 0;
}
Compiling this yields:
$ g++ -std=c++17 -W -Wall -Wextra main.cc
/tmp/ccfexx8L.o: In function `Foo::Foo(Foo const&)':
main.cc:(.text._ZN3FooC2ERKS_[_ZN3FooC5ERKS_]+0x35): undefined reference to
`std::vector<int, std::allocator<int> >::vector(std::vector<int,
std::allocator<int> > const&)'
It seems this is related to the 0-sized array member, which I realise is
non-standard (https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html). Making the
array member size 1 results in the relevant vector copy constructor being
generated, as does inserting the following otherwise-useless code into the
translation unit:
static inline std::vector<int> dummy(const std::vector<int> &p) {
return p;
}
This behaviour surprised me because I expected GCC to either (1) emit a compile
error or (2) generate the relevant constructors. Is this known/expected
behaviour?
Possibly of relevance PR 42121 and PR 14799.
