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

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:开发者_JS百科 '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.

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.

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).

To avoid such a mistake, just remember one rule: If you declare an anonymous object with one argument, just place it into a pair of parentheses!