VS2013 default initialization vs value initialization

Question

Consider the code below

struct B
{
    B() : member{}{};
    int member[10];
};

int main()
{
    B b;
}

VS2013 compiler gives the following warning:

warning C4351: new behavior: elements of array 'B::member' will be default initialized 1> test.vcxproj -> C:\Users\asaxena2\documents\visual studio 2013\Projects\test\Debug\test.exe

This is documented here

With C++11, and applying the concept of 'default initialization', means that elements of B.member will not be initialized.

But I believe that member{} should perform value initialization and not default initialization. Is the VS2013 compiler broken?

$8.5/6

To default-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.
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.

$8.5.1

List-initialization of an object or reference of type T is defined as follows:
— If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.
— Otherwise, if T is an aggregate, aggregate initialization is performed (8.5.1).

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from an empty initializer list (8.5.4). [ Example:

  struct S { int a; const char* b; int c; };
  S ss = { 1, "asdf" };

initializes ss.a with 1, ss.b with "asdf", and ss.c with the value of an expression of the form int(), that is, 0. —end example ]

Answer 1

It seems to be an incorrectly worded warning message (and I'm surprised it is printing a warning in the first place), but the behavior is correct. B::member is being value initialized, which for an array of int turns into zero initialization. This can be demonstrated using the following:

#include <iostream>

struct B
{
    B() : member{}{};
    int member[10];
};

struct C
{
    C() {};
    int member[10];
};

int main()
{
    B b;
    for(auto const& a : b.member) std::cout << a << ' ';
    std::cout << std::endl;

    C c;
    for(auto const& a : c.member) std::cout << a << ' ';
    std::cout << std::endl;
}

If you compile and run in Debug mode this results in the output:

0 0 0 0 0 0 0 0 0 0
-858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460

The numbers in the second line are 0xCCCCCCCC, the debug pattern the VC++ compiler fills memory with in Debug mode. Thus B::member is being zero-initialized, while no initialization is performed for C::member.

Disclaimer: I know that reading from an uninitialized variable is undefined behavior, but this is the best proof I could come up with.

Answer 2

The compiler warning is incorrect; it is actually performing value-initialization as required by the standard.

Example:

#include <iostream>

struct B {
    B() : member{}{};
    int member[10];
};

int main() {
    int a[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    B &b = *new (a) B;
    std::cout << b.member[9];  // prints '0'
}