[Bug objc++/59117] New: Is the default constructor generated by the compiler actually called?

["konevaanna2012 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59117

            Bug ID: 59117
           Summary: Is the default constructor generated by the compiler
                    actually called?
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: objc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: konevaanna2012 at gmail dot com

Dear sirs!
I am Anna Koneva from Moscow, Russia. I have faced with some problem using
XCode 4.6.3 and the LLVM GCC 4.2. I use the following code.

Let's say we have an Objective-C class and a C++ class:

FileObjCClass.h:

#include "CppClass.h"

@interface ObjCClass :NSObject
{
    CppClass cppClass;
}

- (void)SomeObjCMethod;
@end

File ObjCClass.mm:

#import "ObjCClass.h"

@implementation ObjCClass

- (void)SomeObjCMethod
{
    cppClass.someMethod(43);
}
@end

FileCppClass.h:

classCppClass
{
private:
    int _var;
public:
    CppClass();
    int someMethod(int param);
};

File CppClass.cpp:

#include "CppClass.h"
#include <iostream>

using namespace std;

CppClass::CppClass()
{
    _var = 26;
}

int CppClass::someMethod(int param)
{
    cout << "param = " << param << endl << "_var = " << _var << endl;
    return param;
}

File main.mm:

#import <Foundation/Foundation.h>
#import "ObjCClass.h"

int main(int argc, const char * argv[])
{
    ObjCClass *obj = [[ObjCClass alloc]init];
    [obj SomeObjCMethod];
    [obj release];
    return 0;
}

If you switch to ÂLLVM GCC from ÂApple LLVM compiler (selected by default
after the creation of the project) a checkbox appears in the project settings
(which is not available for Clang) ÂCall C++ Default Ctors / Dtors in
Objective-CÂ (GCC_OBJC_CALL_CXX_CDTORS, -fobjc-call-cxx-cdtors).

Case 1. The checkbox is off.

Results: compiler warnings

ÂType 'CppClass' has a user-defined constructorÂ
ÂC++ constructors and destructors will not be invoked for Objective-C fieldsÂ

The sequence of calls (in order from top to bottom):
â [ObjCClass init]
â [ObjCClass SomeObjCMethod]
CppClass::someMethod(int)

What is interesting, the program succeeds and does not crash.

In this case, our constructor (CppClass::CppClass) actually is not called, but
the object is still created. It seems that the constructor generated by the
compiler is called, but the ours is ignored. 

So my question - is it really called the constructor generated by GCC? How can
I prove it? Can I see it in the assembly listing or somehow else? Or am I
totally wrong?

In this case the variable _var has a value 0 both in release and in debug
modes, though in C++ member variables are not initialized by default.

Case 2. If the checkbox is on, then our constructor will be successfully
invoked.

Results:
no compiler warnings

The sequence of calls:
_objc_rootAllocWithZone
object_cxxConstructFromClass(objc_object*, objc_class*)
â [ObjCClass .cxx_construct]
CppClass::CppClass()
â [ObjCClass init]
â [ObjCClass SomeObjCMethod]
CppClass::someMethod(int)