Why is gcc and msvc allow const objects without user-provided constructor? Is this a bug?

Question

This code:

class A {
public:
    A() = default;
};

int main()
{
   const A a;

   return 0;
}

can be compiled without errors in Visual C++ (from VS 2015 Update 2) and GCC (5.3.1), but can not be compiled with Clang, it gives this error:

test.cpp:8:13: error: default initialization of an object of const type
      'const A' without a user-provided default constructor
           const A a;
                   ^
                    {}
1 error generated.

Who is right, clang or gcc and MSVC? Are other compilers behavior is a bug?

PS. const A a{}; is compiled without errors with all three. Same if we define empty constructor: A::A() {}.

PPS. This is NOT a duplicate. I emphasize the actual question.

Update Filed a bug in MSVC: https://connect.microsoft.com/VisualStudio/feedback/details/2538005 S.T.L. said that they are in process of fixing them all to conform clang tests: https://twitter.com/StephanTLavavej/status/715923311796953089

Why you define the default constructor to default? If you what it default dont declareit, or declare it and define it. — Asier Sánchez Rodríguez, Mar 31 '16 at 11:24
@AsierSánchezRodríguez Because I can. This is not the question about. — vladon, Mar 31 '16 at 11:25
How about [this one](http://stackoverflow.com/questions/28337951/clang-compile-error-with-default-initialization?lq=1)? — TartanLlama, Mar 31 '16 at 11:44

score 2 · Accepted Answer · edited May 23 '17 at 11:44

gcc is conforming to DR253; Why does gcc allow a const object without a user-declared default constructor but not clang? Note that gcc does not require the presence of the explicitly-defaulted default constructor:

struct A {};  // No uninitialized members
int main() {
   const A a; // OK with gcc
}

MSVC is non-conformant in that it will allow compilation of the code when A has an uninitialized data member, regardless of whether there is an explicitly-defaulted default constructor, but it does at least issue a diagnostic (C4269) so it should be easy to make conformant:

struct A { int i; }; // Uninitialized data member
int main() {
   const A a;  // Should be ill-formed; MSVC accepts with diagnostic
}

main.cpp(8): warning C4269: 'a': 'const' automatic data initialized with compiler generated default constructor produces unreliable results

clang has a patch accepted and ready to land (since June 2015) but it appears to have been forgotten.

Has DR253 made the standard yet? Last I checked (October '15) there was no activity on it, and it was still in "Drafting" status. — Michael Foukarakis, Mar 31 '16 at 11:43

Why is gcc and msvc allow const objects without user-provided constructor? Is this a bug?

1 Answers1