Question

I have a class and a const variable.

struct A 
{
    int b;
};

A const a;

The class A is POD and can be initialized like this.

A const a = { 3 };

IMHO, it looks fine to have a constructor like this.

struct A 
{
    int b;

    A(int newB) : b(newB)
    {
    }
};

But Clang assumes A as non-aggregate type. Why I can’t have constructor like that? Or should I do something else?

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I modified question to present my original meaning. I had wrote the struct as class by mistake, and sorry for @Johannes about confusing 🙂

Answer 1

POD means Plain Old Data type which by definition cannot have user-defined constructor.

POD is actually an aggregate type (see the next quotation). So what is aggregate? The C++ Standard says in section §8.5.1/1,

An aggregate is an array or a class
(clause 9) with no user-declared
constructors (12.1), no private or
protected nonstatic data members
(clause 11), no base classes (clause
10), and no virtual functions (10.3).

And section §9/4 from the C++ Standard says,

[….] A POD-struct is an aggregate class that has no non-static data
members of type non-POD-struct,
non-POD-union (or array of such types)
or reference, and has no user-defined
copy assignment operator and no
user-defined destructor. Similarly, a
POD-union is an aggregate union that
has no non-static data members of type
non-POD-struct, non-POD-union (or
array of such types) or reference, and
has no user-defined copy assignment
operator and no user-defined
destructor. A POD class is a class
that is either a POD-struct or a
POD-union.

From this, its also clear that POD class/struct/union though cannot have user-defined assignment operator and user-defined destructor also.

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There are however other types of POD. The section §3.9/10 says,

Arithmetic types (3.9.1),
enumeration types, pointer types, and
pointer to member types (3.9.2), and
cv-qualified versions of these types
(3.9.3) are collectively called scalar
types. Scalar types, POD-struct types,
POD-union types (clause 9), arrays of
such types and cv-qualified versions
of these types (3.9.3) are
collectively called POD types.

Read this FAQ : What is a “POD type”?