Clang vs GCC: Single-colon in Enum usage

Question

The following code compiles with g++ -std=c++11 but not with clang++ -std=c++11.

Questions

1. What is the meaning of the single colon "operator" in this context?
   • Clarification/Edit: How does GCC interpret the code?
2. How can I make GCC not compile this code? (Assuming that Clang follows the C++ Standard here.) Is there a flag for this?

Code

Compile with g++ -std=c++11 main.cpp and clang++ -std=c++11 main.cpp. I am using GCC 4.8 and Clang 6.0.0 (trunk).

#include <iostream>
#include <vector>

enum Dir { LEFT, RIGHT };

int main(int argc, char** argv) {
  // Interesting line: Notice the single ':'
  std::vector<Dir> dirs = { Dir:LEFT, Dir:RIGHT };

  for (auto v: dirs) {
    std::cout << v << std::endl;
  }
  return 0;
}

Clang Error Message

For completeness and search-ability:

 $ clang++ -std=c++11 main.cpp                                                                                                                                                                                            

main.cpp:7:29: warning: use of GNU old-style field designator extension [-Wgnu-designator]
  std::vector<Dir> dirs = { Dir:LEFT, Dir:RIGHT };
                            ^~~~
                            .Dir = 
main.cpp:7:39: warning: use of GNU old-style field designator extension [-Wgnu-designator]
  std::vector<Dir> dirs = { Dir:LEFT, Dir:RIGHT };
                                      ^~~~
                                      .Dir = 
main.cpp:7:20: error: no matching constructor for initialization of 'std::vector<Dir>'
  std::vector<Dir> dirs = { Dir:LEFT, Dir:RIGHT };
                   ^      ~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:269:7: note: candidate constructor not viable: cannot convert argument of incomplete type 'void' to 'std::vector::size_type' (aka 'unsigned long') for 1st argument
      vector(size_type __n, const allocator_type& __a = allocator_type())
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:281:7: note: candidate constructor not viable: cannot convert argument of incomplete type 'void' to 'std::vector::size_type' (aka 'unsigned long') for 1st argument
      vector(size_type __n, const value_type& __value,
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:331:7: note: candidate constructor not viable: cannot convert argument of incomplete type 'void' to 'const std::vector<Dir, std::allocator<Dir> >' for 1st argument
      vector(const vector& __x, const allocator_type& __a)
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:340:7: note: candidate constructor not viable: cannot convert argument of incomplete type 'void' to 'std::vector<Dir, std::allocator<Dir> >' for 1st argument
      vector(vector&& __rv, const allocator_type& __m)
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:364:7: note: candidate constructor not viable: cannot convert argument of incomplete type 'void' to 'initializer_list<std::vector<Dir, std::allocator<Dir> >::value_type>'
      (aka 'initializer_list<Dir>') for 1st argument
      vector(initializer_list<value_type> __l,
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:392:9: note: candidate template ignored: substitution failure [with _InputIterator = void]: no type named 'iterator_category' in 'std::iterator_traits<void>'
        vector(_InputIterator __first, _InputIterator __last,
        ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:256:7: note: candidate constructor not viable: requires single argument '__a', but 2 arguments were provided
      vector(const allocator_type& __a)
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:310:7: note: candidate constructor not viable: requires single argument '__x', but 2 arguments were provided
      vector(const vector& __x)
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:327:7: note: candidate constructor not viable: requires single argument '__x', but 2 arguments were provided
      vector(vector&& __x) noexcept
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:248:7: note: candidate constructor not viable: requires 0 arguments, but 2 were provided
      vector()
      ^
2 warnings and 1 error generated.

Answer 1

To cause gcc to reject the code, use -pedantic switch.

The colon is an extension in GNU mode: X:Y means .X = Y, which is a designated initializer. (Neither of these are supported in ISO C++).

---

gcc also accepts the following code:

std::vector<int> v = { .a = 1, .b = 2 };

but rejects the code:

struct S { int p, q; S() {} };
S s = { .a = 1, .b = 2 };   // S has no member named 'a'

I guess this is a compiler bug; something about initializing a std::vector<int> causes it to ignore the names of the designated initializers. Note, this sort of thing is a hallmark of non-standard features: often the very reason they aren't in the standard is that they didn't mix well with other language features and nobody could come up with a sensible proposal to handle all possible cases.