Call virtual functions from a reference

Question

I have a structure like this

struct structure
{
    BaseObject &A; //BaseObject has a function declared as virtual
};

In run time, I am dynamically assign an object to &A

structure *s = new structure;
DerivedObjectB *B = new DerivedObjectB(); //Derived class overloads the virtual function 
s->A = *B; //s is a pointer of the structure. S has been initialized

I can compile this code but I am getting a seg-fault error in runtime. I have a restriction that I can not use a pointer. This is not a homework. The compiler I am using as a reverse compiler has a restriction of using pointers because of an issue in building SSA

That code cant compile, references are not assignable. Show us real code. — K-ballo, Jun 12 '12 at 17:31
"*I have a restriction that I can not use a pointer.*" Who would give you such a silly restriction? Is this homework? — ildjarn, Jun 12 '12 at 17:33
@Luchian Grigore: True, it will compile but not do what the OP expects. — K-ballo, Jun 12 '12 at 17:33
This is not a homework. The compiler I am using as a reverse compiler has a restriction of using pointers because of an issue in building SSA — codereviewanskquestions, Jun 12 '12 at 17:34
And `s` is a valid pointer that points to a struct with valid reference to a live object? :-) — Bo Persson, Jun 12 '12 at 17:38
Might be relevant: http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c — anatolyg, Jun 12 '12 at 17:47
The code is ill-formed because `structure` has no default constructor; the reference type data member inhibits the implicit declaration of a default constructor (in C++11, its implicitly declared default constructor is defined as deleted). — James McNellis, Jun 12 '12 at 17:57
@James : If I'm not mistaken, `structure` is an aggregate type, so the following should work as-is: `BaseObject a; structure s = { a };`. — ildjarn, Jun 12 '12 at 18:49
@ildjarn: Yes, aggregate initialization would work. `new structure` won't, though, because of the missing default constructor. — James McNellis, Jun 12 '12 at 18:56
@James : Ah, I see what you mean. Somehow I thought you were referring to the definition of `structure` itself as ill-formed. Sorry for the noise. — ildjarn, Jun 13 '12 at 00:46

score 2 · Accepted Answer · answered Jun 12 '12 at 18:00

If you cannot use a pointer, you must use a reference. If you use a reference, it must be initialized to it's final value as it's constructed.

struct structure
{
    BaseObject &A; //BaseObject has a function declared as virtual
    structure(BaseObject &A_) : A(A_) {}
};

DerivedObjectB *B = new DerivedObjectB(); 
structure *s = new structure(*B); //done
//keep in mind you cannot delete `B` until _after_ you delete `s`

Your code above shouldn't have compiled since a structure could not be created that way since it had no automatic default structure since it has a reference member. Also, Even if it did compile, the A member would have been a BaseObject copy of the DerivedObjectB's parent BaseObject object, or some other bizzare not-what-you-wanted.

Are you absolutely sure you cannot use pointers there? That makes no sense at all, and makes this very difficult to work with.

score 1 · Answer 2 · answered Jun 12 '12 at 17:52

You cannot change object referenced by A, once structure is constructed. What you probably would like to have is:

struct structure
{
  structure() : A(NULL) {}
  ~structure() { if (this->A) { delete this->A; } }
  BaseObject * A;
};

structure *s = new structure;
s->A = new DerivedObjectB();

Anyway, using raw pointers is error prone and you should consider it.

score 0 · Answer 3 · answered Jun 12 '12 at 17:38

0

I'm betting you just declared

structure* s;

or

structure* s = NULL;

amirite? ~~Otherwise, there's no reason for the crash.~~

Or s->A references an object that went out of scope, so when you call operator = on it you get into undefined behavior.

It's one of these two.

answered Jun 12 '12 at 17:38

Luchian Grigore

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thanks for your answer. Since I initialized the structure I guess its the second case. – codereviewanskquestions Jun 12 '12 at 17:41

score 0 · Answer 4 · answered Jun 12 '12 at 17:57

0

This should not even compile. The reference cannot be rebound after construction, and you should have received an error telling you that it must have been bound to something.

answered Jun 12 '12 at 17:57

Puppy

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It's not even bound at construction. – R. Martinho Fernandes Jun 12 '12 at 18:00
1

@Greg Either that is a bug or a GNU extension (I hope it's not the latter). Try with a more recent compiler and without crazy extensions. – R. Martinho Fernandes Jun 12 '12 at 18:03
I'm as surprised as you are. Explanation: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39045 – Greg Jun 12 '12 at 18:05

Call virtual functions from a reference

4 Answers4