How to include C++ headers in an Objective C++ header?

The problem is that you have to avoid all C++ semantics in the header to allow normal Objective-C classes to include it. This can be accomplished using opaque pointers.

CPPClass.h

class CPPClass
{
public:
    int getNumber()
    {
        return 10;
    }
};

ObjCPP.h

//Forward declare struct
struct CPPMembers;

@interface ObjCPP : NSObject
{
    //opaque pointer to store cpp members
    struct CPPMembers *_cppMembers;
}

@end

ObjCPP.mm

#import "ObjCPP.h"
#import "CPPClass.h"

struct CPPMembers {
    CPPClass member1;
};

@implementation ObjCPP

- (id)init
{
    self = [super init];
    if (self) {
        //Allocate storage for members
        _cppMembers = new CPPMembers;

        //usage
        NSLog(@"%d", _cppMembers->member1.getNumber());
    }

    return self;
}

- (void)dealloc
{
    //Free members even if ARC.
    delete _cppMembers;

    //If not ARC uncomment the following line
    //[super dealloc];
}

@end