Do I really need to implement user-provided constructor for const objects?

c++, c++11, constants, constructor, language-lawyer

Solution

N3797 §8.5/7 says:

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

There's no further example or explanation of this. I agree it seems pretty bizarre. Furthermore the rule was updated in C++11 to be more restrictive than it was in C++03, when class types needed user-declared constructors. (Your constructor is user-declared.)

The workaround is be to ask for value initialization using `{}`, or use Dietmar's clever out-of-class `inline` definition.

GCC does provide a diagnosis (and quite a nice one, referring to the newer C++11 requirements) if you add another member without an initializer.

  private:
    int i = 1;
    int j;

unmem.cpp:11:11: error: uninitialized const ‘a’ [-fpermissive]
   const A a;
           ^
unmem.cpp:1:7: note: ‘const class A’ has no user-provided default constructor
 class A {
       ^
unmem.cpp:3:5: note: constructor is not user-provided because it is explicitly defaulted in the class body
     A() = default;
     ^
unmem.cpp:7:9: note: and the implicitly-defined constructor does not initialize ‘int A::j’
     int j;

The GCC source refers to DR 253, Why must empty or fully-initialized const objects be initialized? This is an open issue in the standard, last updated in August 2011 (post-C++11) with this note:

If the implicit default constructor initializes all subobjects, no initializer should be required.

Therefore whereas Clang complies with C++11 (and will comply as-is with C++14), GCC is implementing the latest thinking of the standardization committee.

Filed a GCC bug. I predict that you'll need `-pedantic` to get a diagnosis when (and if) the bug is fixed.

Problem

I have the code: ``` class A { public: A() = default; private: int i = 1; }; int main() { const A a; return 0; } ``` It compiles fine on g++ (see ideone), but fails on clang++ with error: default initialization of an object of const type 'const A' requires a user-provided default constructor I reported this issue on LLVM bug-tracker and got it INVALID. I see it absolutly pointless to try to convince the clang developers. On the other side, I don't see the reason for such restriction. Can anyone advise, if the C++11 Standard somehow implies this code to be invalid? Or should I just report a bug to g++? Or maybe there is enough freedom in language rules to handle this code in many ways?

Original source

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