Open Radar

 
Summary:
If you try to instantiate a C++ object as an instance variable in your ObjC class, the error that would normally tell you that a pure virtual method is unimplemented is missing, and the application compiles anyway. This will crash at runtime instead.

Steps to Reproduce:
1. Implement an abstract base class in C++
2. Implement a concrete sub class in C++
3. Accidentally mistype or miss to override one or more of the pure abstract virtual method from super
4. Compile

Expected Results:
5. Build error, refusing to build since you have a method missing

Actual Results:
5. Application compiles without errors, but crashes at runtime when the missing method is called.

Regression:
The error works fine for C++ ivars, as well as for stack and heap allocations of the C++ object.

Notes:
This is a very easy mistake to make in C++, only slightly mitigated by 'override' since there is so much old code that doesn't use override yet. A single type change in super's signature will break the overloading of that method. This has caused several very serious crashers in our iOS app.

Sample code, also attached:

#import <Foundation/Foundation.h>

class CppAbstractBase {
public:
    virtual void testA() = 0;
    virtual void testB() = 0;
    int a;
};

class CppConcreteSub : public CppAbstractBase {
    virtual void testA() { }
};

@interface Objc : NSObject {
    CppConcreteSub _concrete; // <-- SHOULD ERROR
}
- (CppAbstractBase*)abstract;
@end
@implementation Objc
- (CppAbstractBase*)abstract {
    return &_concrete;
}
@end

class Cpp {
public:
    CppConcreteSub sub; // <-- ERRORS CORRECTLY
    // Field type 'CppConcreteSub' is an abstract class
};

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        CppConcreteSub sub; // <-- ERRORS CORRECTLY
        // Variable type 'CppConcreteSub' is an abstract class, Unimplemented pure virtual method 'testB' in CppConcreteSub
        
        Objc *objc = [Objc new];
        [objc abstract]->testB(); // <-- CRASHES HORRIBLY
        
        // insert code here...
        NSLog(@"Hello, World!");
        
    }
    return 0;
}