# Implementation divergence with const int i; and mutable

While teaching the other day, I had just finished talking about one place in which C++ departs from its usual no-nannying philosophy: C++ prevents the programmer from writing

const int i;


“I just want an entity with decltype const int! I don’t care about its value. I’m just trying to test if f(i) compiles when i is const. I’m not going to read from i.” Unfortunately, both the paper standard and every implementation — Clang, GCC, MSVC, and ICC respectively — forbid this construction.

error: default initialization of an object of const type
'const int'

error: uninitialized 'const i' [-fpermissive]

error C2734: 'i': 'const' object must be initialized
if not 'extern'

error: const variable "i" requires an initializer


(Incidentally, GCC, what happened to const i’s type in that error message?)

The same thing applies to types with default-initialized int fields. For example,

struct A { int i; };
const A a;


Clang, GCC, MSVC, ICC?

error: default initialization of an object of const type
'const A' without a user-provided default constructor

error: uninitialized 'const a' [-fpermissive]

(MSVC: contented silence)

error: const variable "a" requires an initializer --
class "A" has no user-provided default constructor


We went on to talk about the opposite of const, which is mutable. And then a student asked me a question I hadn’t thought of before.

Since mutable makes the field non-const, does that mean it’s okay again to define it without an initializer?

That is:

struct B { mutable int i; };
const B b;


Clang, GCC, MSVC, ICC?

(Clang: contented silence)

error: uninitialized 'const b' [-fpermissive]

(MSVC: contented silence)

error: const variable "b" requires an initializer --
class "B" has no user-provided default constructor


It is not clear to me what the intended behavior of the core language is, here.

(Of course what I would like to happen is for C++ to stop its overparenting and eliminate the error even in the simplest case. This would also simplify the C++17 standard to the tune of one sentence. In fact, C++2a instead balloons that one sentence into a whole paragraph, with a cross-reference to CWG 253. WG21 never met a simplification they couldn’t turn into an opportunity for expansion! See also P1155 “More Implicit Moves” suggested wording versus what actually happened in C++2a.)

Intel ICC (which reports __EDG_VERSION__ == 500) actually has another interesting quirk: it seems to treat constructors implicitly generated due to non-static data member initializers as fundamentally different animals from regular defaulted constructors. That is, EDG produces a warning (but not a hard error) for this code:

struct C { int i = 42; };
const C c;


Clang, GCC, MSVC, ICC?

(Clang: contented silence)

(GCC: contented silence)

(MSVC: contented silence)

warning #854: const variable "c" requires an initializer --
class "C" has no user-provided default constructor


Adding C() = default; shuts up EDG’s warning.

Posted 2019-12-04