Value initialization: default initialization or zero initialization?

问题

I have templated gray_code class which is meant to store some unsigned integer whose underlying bits are stored in Gray code order. Here it is:

template<typename UnsignedInt>
struct gray_code
{
    static_assert(std::is_unsigned<UnsignedInt>::value,
                  "gray code only supports built-in unsigned integers");

    // Variable containing the gray code
    UnsignedInt value;

    // Default constructor
    constexpr gray_code()
        = default;

    // Construction from UnsignedInt
    constexpr explicit gray_code(UnsignedInt value):
        value( (value >> 1) ^ value )
    {}

    // Other methods...
};

In some generic algorithm, I wrote something like this:

template<typename UnsignedInt>
void foo( /* ... */ )
{
    gray_code<UnsignedInt> bar{};
    // Other stuff...
}

In this piece of code, I expected bar to be zero-intialized and therefore bar.value to be zero-initialized. However, after having struggled with unexpected bugs, it appears that bar.value is initialized with garbage (4606858 to be exact) instead of 0u. That surprised me, so I went to cppreference.com to see what the line above was exactly supposed to do...

From what I can read, the form T object{}; corresponds to value initialization. I found this quote interesting:

In all cases, if the empty pair of braces {} is used and T is an aggregate type, aggregate-initialization is performed instead of value-initialization.

However, gray_code has a user-provided constructor. Therefore it is not an aggregate thus aggregate initialization is not performed. gray_code has no constructor taking an std::initializer_list so list initialization is not performed either. The value-initialized of gray_code should then follow the usual C++14 rules of value initialization:

1) If T is a class type with no default constructor or with a user-provided default constructor or with a deleted default constructor, the object is default-initialized.

2) If T is a class type without a user-provided or deleted default constructor (that is, it may be a class with a defaulted default constructor or with an implicitly-defined one) then the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor.

3) If T is an array type, each element of the array is value-initialized.

4) Otherwise, the object is zero-initialized.

If I read correctly, gray_code has an explicitly defaulted (not user-provided) default constructor, therefore 1) does not apply. It has a defaulted default constructor, so 2) applies: gray_code is zero-initialized. The defaulted default constructor seems to meet all the requirements of a trivial default constructor, so default initialization should not happen. Let's have a look then at how gray_code is zero-initialized:

If T is a scalar type, the object's initial value is the integral constant zero implicitly converted to T.

If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.

If T is a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits.

If T is array type, each element is zero-initialized

If T is reference type, nothing is done.

gray_code is a non-union class type. Therefore, all of its non-static data members should be initialized which means that value is zero-initialized. value satisfies std::is_unsigned and is therefore a scalar type, which means that it should be initialized with "the integral constant zero implicitly converted to T".

So, if I read correctly all of that, in the function foo above, bar.value should always be initialized with 0 and it should never be initialized with garbage, am I right?

Note: the compiler I compiled my code with is MinGW_w4 GCC 4.9.1 with (POSIX threads and dwarf exceptions) in case that helps. While I sometimes get garbage on my computer, I never managed to get anything else than zero with online compilers.

Update: It seems ~~to be a GCC bug~~ that the error is mine and not that of my compiler. Actually, when writing this question, I assumed for the sake of simplicity that

class foo {
    foo() = default;
};

and

class foo {
    foo();
};

foo::foo() = default;

were equivalent. They are not. Here is the quote from the C++14 standard, section [dcl.fct.def.default]:

A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

In other words, when I got garbage values, my defaulted default constructor was indeed user-provided since it was not explicitly efaulted on its first declaration. Therefore, what happened was not zero initialization but default initialization. Thanks @Columbo again for pointing out the real problem.

回答1:

So, if I read correctly all of that, in the function foo above, bar.value should always be initialized with 0 and it should never be initialized with garbage, am I right?

Yes. Your object is direct-list-initialized. C++14's* [dcl.init.list]/3 specifies that

List-initialization of an object or reference of type T is defined as follows:

[… Inapplicable bullet points…]

Otherwise, if T is an aggregate, aggregate initialization is performed (8.5.1).

Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

[…]

Your class isn't an aggregate since it has user-provided constructors, but it does have a default constructor. [dcl.init]/7:

To value-initialize an object of type T means:

if T is a (possibly cv-qualified) class type (Clause 9) with either no default constructor (12.1) or a default constructor that is user-provided or deleted, then the object is default-initialized;

if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;

[dcl.fct.def.default]/4:

A special member function is user-provided if it is user-declared and not explicitly defaulted […] on its first declaration.

So your constructor is not user-provided, therefore the object is zero-initialized. (The constructor is not called since its trivial)

And finally, in case this was not clear, to zero-initialize an object or reference of type T means:

if T is a scalar type (3.9), the object is initialized to the value obtained by converting the integer literal 0 (zero) 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;

[…]

Thus either