Weird GCC array initialization behavior

Question

I encountered a variant of this code when looking at another question (the original code used a std::thread instead of std::vector, but the syntax is the same):

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>

int main()
{
    std::vector<double> vecs[10] = std::vector<double>(10, 1);
    for(auto& vec: vecs){
        std::copy(vec.begin(), vec.end(), std::ostream_iterator<double>(std::cout, " "));
        std::cout<<std::endl;
    }
    return 0;
}

This code shouldn't compile; std::vector<double> vecs[10] = std::vector<double>(10, 1); is not valid initialization syntax, and clang rejects it with error: array initializer must be an initializer list. However, GCC accepts it and appears to initialize every vector in the list with a copy of the specified temporary.

Is this some GCC extension I've never heard about (that somehow also managed to survive -pedantic-errors) or just a plain bug?

@JonathonReinhart Both are in C++11 mode, and AFAIK that syntax has never been legal in C++. — T.C.
– T.C., Commented Jun 24, 2014 at 5:39
Is this not the (2) fill constructor, as mentioned here? Edit: Ah nevermind, missed the array in your variable declaration. — Jonathon Reinhart
– Jonathon Reinhart, Commented Jun 24, 2014 at 5:42
@JonathonReinhart Yes, that's a valid constructor, the problem is the attempt to use that to initialize the array. That should not compile. — T.C.
– T.C., Commented Jun 24, 2014 at 5:44
Minimal example: struct a {a();} x[2] = a();. gcc compiles it. Without the constructor declaration, it won't. Clearly a bug. — n. m. could be an AI
– n. m. could be an AI, Commented Jun 24, 2014 at 17:35

choeger · Accepted Answer · 2014-06-24 17:08:45Z

4

I would consider this a bug.

#include <vector>

int main()
{
  std::vector<double> x = std::vector<double>(10, 1);
  std::vector<double> vecs[10] = x;
  return 0;
}

Works (as you have spotted).

While

int main()
{
  int x = 10;
  int is[10] = x;
  return 0;
}

yields the (expected) error.

answered Jun 24, 2014 at 17:08

choeger

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T.C. · Accepted Answer · 2014-06-25 04:22:53Z

Further investigation:

struct A { A() { } };
int main() { A a[10] = A(); }

This compiles in GCC.

struct A { A() = default; };
int main() { A a[10] = A(); }

Also compiles in GCC 4.9 but not earlier versions I tested (4.6-4.8).

struct A { };
int main() { A a[10] = A(); }

Doesn't compile.

struct B { virtual ~B() { } };
struct A : B { };
int main() { A a[10] = A(); }

Compiles.

struct B {  ~B() { } };
struct A : B { };
int main() { A a[10] = A(); }

Doesn't compile.

I think it's safe to say that it's a bug. No sane extension would have this sort of behavior. Note that A in both the second case and the third case are POD types (the only difference being an explicitly defaulted default constructor), but they are treated differently by GCC 4.9.

Edit: Going through the GCC bugzilla again, this bug report appears to be related. Looks like I missed it the first time through because the title was talking about string literals.

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