What's the c++ inline class?

Question

I accidentally found that the Clang compiler allows :

inline class AAA
{
};

in C++. What's this?

---

PS. I reported this to Clang mailing list [email protected], and now waiting for reply. I'll update this question by I'm informed.

Solution 1

I got an answer from Clang mailing list. It was a bug: http://llvm.org/bugs/show_bug.cgi?id=3941

However it looks already been fixed in recent build. Thanks anyway :)

Here's the conversation: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-March/014207.html

Solution 2

It's allowed in case you wish to declare a function that returns an object of that class directly after the class' declaration, for example :

#include <iostream>

inline class AAA 
{
public:
    AAA()
    {
        // Nothing
    }

    AAA(const AAA& _Param)
    {
        std::cout << "Calling Copy Constructor of AAA\n";
    }
}A()
 {
     AAA a;
     return a;
 };

int main()
{
    A();
    return 0;
}

Also you should notice the compiler errors (or warnings) that appear in other illegal cases, such as declaring a variable instead of A(), also notice that the compiler states that it ignores the inline keyword if you didn't declare any function.

Hope that's helpful.

Edit : For The comment of Eonil

If you are talking about your code above in the question, then it's the same case as I see, the compiler will give you a warning : 'inline ' : ignored on left of 'AAA' when no variable is declared

However, if you use the code in my answer but replace A() with a variable, B for example, it will generate a compiler error : 'B' : 'inline' not permitted on data declarations

So we find that the compiler made no mistake with accepting such declarations, how about trying to write inline double; on its own? It will generate a warning : 'inline ' : ignored on left of 'double' when no variable is declared

Now how about this declaration :

double inline d()
{
}

It gives no warnings or errors, it's exactly the same as :

inline double d()
{
}

since the precedence of inline is not important at all.

The first code (in the whole answer) is similar to writing :

class AAA
{
    // Code
};

inline class AAA A()
{
    // Code
}

which is legal.

And, in other way, it can be written as :

class AAA
{
    // Code
};

class AAA inline A()
{
    // Code
}

You would be relieved if you see the first code (in the whole answer) written like :

#include <iostream>

class AAA 
{
    // Code
} inline A()
 {
    // Code
 };

But they are the same, since there is no importance for the precedence of inline.

Hope it's clear and convincing.

Solution 3

clang shouldn't allow this, inline can only be used in the declaration of functions, from ISO/IEC 14882:2003 7.1.2 [dcl.fct.spec] / 1 :

Function-specifiers can be used only in function declarations.

inline is one of three function-specifiers, virtual and explicit being the others.

As @MatthieuM notes, in the next version of C++ (C++0x), the inline keyword will also be allowed in namespace definitions (with different semantics to inline as a function-specifier).

Comments

[Matthieu M.]

In the context of C++0x, it can be used with namespaces too.

[Xeo]

@Matthieu: Could you expand on that?

[Xeo]

My eyes!! +1 though for a nice and cryptic example when this works. :)

[polemon]

Could you elaborate WTF, is happening here? Why would I ever declare a function like that, and is that function a method of the class?

[Tamer Shlash]

@polemon: This function is not a method of that class, it's a function on its own, you could declare funciton or variable immediately after the closing brace and before the semicolon ; and it (the function) will return an object of that class (and the variable will be an object of that class). And for reason, I don't actually have any reason for functions, there may be some reasons for variables, yet I don't have any of them in mind right now.

[Nicholas Knight]

@Mr.TAMER's particular construct might be legal (and it's definitely crazy), but if there's no function, I think the compiler ought to reject it, not warn about it. (And the clang version with Xcode4 does neither.) This still smells like a bug to me. Does anyone have chapter&verse from the standard on this?

[eonil]

Great description. But unfortunately, I tried to compile exactly same code which I instanced. And Clang did compile it without any error. So obviously it's not this case.

[Chef Pharaoh]

@TamerShlash Is this allowed in c++11? I get some linker errors saying undefined reference.

[Admin]

Might be helpful to clarify that "It's allowed in case you wish to declare a inline function..."