What's actually going on in this AnonymousClass(variable) declaration?

Question

Trying to compile:

class AnonymousClass
{
public:
    AnonymousClass(int x)
    {
    }
};


int main()
{
    int x;
    AnonymousClass(x);
    return 0;
} 

generates errors from MSVC:

foo.cpp(13) : error C2371: 'x' : redefinition; different basic types
    foo.cpp(12) : see declaration of 'x'
foo.cpp(13) : error C2512: 'AnonymousClass' : no appropriate default constructor available

g++'s error messages are similar:

foo.cpp: In function ‘int main()’:
foo.cpp:13: error: conflicting declaration ‘AnonymousClass x’
foo.cpp:12: error: ‘x’ has a previous declaration as ‘int x’
foo.cpp:12: warning: unused variable ‘x’

It's easily fixable by giving the AnonymousClass object an explicit name, but what's going on here and why? I presume that this is more declaration syntax weirdness (like the cases described in Q10.2 and Q10.21 of the comp.lang.C++ FAQ), but I'm not familiar with this one.

Answer 1

AnonymousClass(x);

It defines a variable x of type AnonymousClass. That is why you're getting redefinition error, because x is already declared as int.

The parentheses are superfluous. You can add even more braces like:

AnonymousClass(x);
AnonymousClass((x));
AnonymousClass(((x)));
AnonymousClass((((x))));
//and so on

All of them are same as:

AnonymousClass x;

Demo: http://www.ideone.com/QnRKH

---

You can use the syntax A(x) to create anonymous object, especially when calling a function:

int x = 10;
f(A(x));        //1 - () is needed
f(A((((x)))));  //2 - extra () are superfluous

Both line 1 and 2 call a function f passing an object of type A :

• http://www.ideone.com/ofbpR

But again, the extra parentheses are still superfluous at line 2.

Answer 2

You're missing an actual name for your variable/object:

AnonymousClass myclass(x);

Instead of that you could as well write...

AnonymousClass (myclass)(x);

So your line of code results in this:

AnonymousClass (x);

Or more common:

AnonymousClass x;

Why it happens? Brackets are just there for logical grouping ("what belongs together?"). The only difference is, they're forced for arguments (i.e. you can't just write AnonymousClass myclass x).