c++ - C++14 value-initialization with deleted constructor

Question

I have some misunderstanding:

Let's mark default constructor of struct A as deleted:

struct A
{
  A() = delete;
};

The next instruction is well-formed and what's that effect?:

A a{};

From cppreference value initilization:

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.

but then the effect of default initialization is:

If T is a class type, the default constructor is called to provide the initial value for the new object.

Or it's aggregate initialization? Thanks!

question from:https://stackoverflow.com/questions/23882409/c14-value-initialization-with-deleted-constructor

Answer 1

Your struct A is :

• a class type that has:
  • no user-provided constructors¹,
  • no private or protected non-static data members,
  • no base classes,
  • no virtual member functions.

It therefore qualifies as an aggregate type, according to the definition provided by § 8.5.1/1.

Then comes the priority of aggregate initialization over value initialization. The standard says that aggregate initialization has precedence over value intialization (draft N3936, § 8.5.4/3, page 201) (emphasis mine)

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

• 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.
• [... more rules...]

⁽¹⁾ As requested in the comments on why a deleted constructor does not count as user-defined, here is what the standard says (draft N3936, § 8.4.2/5, page 198):

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