CODEWITHSUNDEEP
c++gccvisual-c++clanglanguage-lawyer
vladon
vladon

Reputation: 8401

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

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

Upvotes: 3

Views: 287

Answers (1)

ecatmur
ecatmur

Reputation: 157414

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.

Upvotes: 2

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