Clang vs GCC: using a single column in Enum

Question

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

Questions

• What is the meaning of a single “operator” in this context?
  • Clarification / Editing: How does GCC interpret code?
• How can I get GCC not to compile this code? (Assuming 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 searchability:

  $ 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 have gcc reject the code, use the -pedantic switch.

The double is an extension in GNU mode: X:Y means .X = Y , which is the designated initializer. (None of them 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 assume this is a compiler error; something about initializing std::vector<int> makes it ignore the names of the assigned initializers. Please note that such a thing is a distinctive feature of non-standard functions: often the reason why they do not comply with the standard is that they do not mix well with other language functions, and no one could offer a reasonable offer to handle all possible cases.