CODEWITHSUNDEEP
c++c++11list-initialization
xmllmx
xmllmx

Reputation: 42215

Can list initialization not be used for private members?

struct A
{
private:
    int a, b, c;
};

int main()
{
    A a1{};
    A a2 = {};

    return 0;
}

The code was compiled by VC++ 2012 (with the latest update "Nov 2012 CTP").

I expect a1 and a2 are zero-initialized, but not. a1 and a2 are not initialized.

Why?

Upvotes: 5

Views: 178

Answers (2)

Xeo
Xeo

Reputation: 131789

Note that this answer had a slight rewrite that changed the end result to the opposite of what it was before. Thanks to @David Rodríguez - dribeas for enlightening me. :)

This is a bug. Clang 3.2 trunk and GCC 4.7+ agree too and will zero-initialize the members.

Time for some standardese. Note that T x{}; (or = {}) can be interpreted as either list-initialization or aggregate initialization. A here is not an aggregate because it has private members, and as such can not be initialized by the latter.

§8.5.1 [dcl.init.aggr] p1

An aggregate is an array or a class (Clause 9) with [...] no private or protected non-static data members [...]

This only leaves list-initialization and that will value-initialize both a1 and a2.

§8.5.1 [dcl.init.list] p3

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.

Value-initialization is specified as follows for our specific case:

§8.5 [dcl.init] p7

if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized [...]

And this in turn means that the members should be zeroed out.

Upvotes: 6

Nicol Bolas
Nicol Bolas

Reputation: 473222

You mean Microsoft's Community Tech Preview compiler, which they aren't even confident enough in to call it a beta, has bugs in it? ;)

This is a bug; it should behave as you expect. Please report it as such.

Upvotes: 7

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