Confused by the difference about value-initializtion between C++ 11&14 [duplicate]

Question

Consider the following code:

class A {
public:
    int i;
    A() {}
};

class B {
public:
    A a;
    int i;
};

int main() {
    B* p = new B {};
    std::cout << p->i << " " << p->a.i << "\n";
}

Compiled with -std=c++11 in clang++, p->i turns out to be zero, but p->a.i doesn't. Shouldn't the whole object be zeroed as long as its class doesn't have user-provided constructor?

EDIT: Since there are some extensive discussion in the comments, I think it's better to add some excerpt from the standard here:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.
• if T is an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized.

To zero-initialize an object or reference of type T means:

• if T is a scalar type (3.9), the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;
• if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;
• if T is a (possibly cv-qualified) union type, the object’s first non-static named data member is zero-initialized and padding is initialized to zero bits;
• if T is an array type, each element is zero-initialized;
• if T is a reference type, no initialization is performed.

The second bullet of each applies here.

Answer 1

Clang is correct, per the C++11 standard plus relevant DRs

In the original C++11 specification, B{} would perform value-initialization, resulting in a.i being zero-initialized. This was a change in behavior compared to C++98 for cases like

B b = {};

... which were handled as aggregate initialization in C++98 but treated as value-initialization in C++11 FDIS.

However, the behavior in this case was changed by core issue 1301, which restored the C++98 behavior by mandating that aggregate initialization is used whenever an aggregate is initialized by a braced-init-list. Since this issue is considered a DR, it is treated as de facto applying to earlier revisions of the C++ standard, so a conforming C++11 compiler would be expected to perform aggregate initialization here rather than value-initialization.

Ultimately, it's a bad idea to rely on value-initialization to initialize your data members, especially for a class that has user-provided constructors.

Answer 2

It does indeed look like a bug (or, as pointed out in the comments, behaving according to C++03 despite specifying C++11). In C++11, value-initialisation should zero the members of a before calling its default constructor. Initialisation of B is governed by this rule of 8.5/7

if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.

The zero-initialisation should recursively zero-initialise a per this rule of 8.5/5

if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized

and, of course, zero-initialisation of a should set i to zero.

Answer 3

It is not a compiler bug, it is a bug in your code. The compiler seems to be implementing the C++03 behaviour, but this has crucially changed in C++11.

These are some relevant quotes from the C++03 and C++11 standards

In C++03:

To value-initialize an object of type T means:

— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;

(emphasis mine)

In C++11:

To value-initialize an object of type T means:

— if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

— if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.

and

To zero-initialize an object or reference of type T means:

— if T is a scalar type (3.9), the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;

• if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;

---

Note: The following only applies to C++03:

Either remove A's user-provided constructor, or change it to

A() : i() {}

When you value-initialize a B here,

B* p = new B {};

it value-initializes its data members. Since A has a default constructor, the value-initialization results in a call to that. But that constructor does not explicitly initialize A::i, so it gets default-initialized, which for an int means no initialization is performed.

If you had not provided a default constructor for A, then the data member would get zero-initialized when an A is value-initialized.