Ban pointers or references on classes?

This is a discussion on Ban pointers or references on classes? within the C++ Programming forums, part of the General Programming Boards category; Is it possible to ban the use of references or pointers on specific classes? Consider the following: Code: void foo(pp<int>&) ...

  1. #1
    C++まいる!Cをこわせ! Elysia's Avatar
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    Ban pointers or references on classes?

    Is it possible to ban the use of references or pointers on specific classes?
    Consider the following:
    Code:
    void foo(pp<int>&) // pp = CMemoryManager
    {
    }
    
    void foo(pp<int>*) // pp = CMemoryManager
    {
    }
    
    int main()
    {
    ppnew<int> pTest; // Creates a new class of type CMemoryManager
    foo(pTest); // I want to ban this, if possible.
    foo(&pTest); // This too, maybe.
    }
    Is it possible to ban such usage by doing something with the class? If possible, I want it only to be passed by value.

  2. #2
    The superhaterodyne twomers's Avatar
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    Why would you only want it passed by value, and why would you want to restrict its pass type?

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Because it's a memory manager - it keeps a reference count. For every function you pass it to, it should increase its ref count.
    And it will create less bugs and such and for fun, too.

  4. #4
    and the hat of sweating
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    If you're talking about the class constructor function, you can use the explicit keyword, otherwise you can make a function private so it can't be called.

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    I don't know how explicit works, nor what implicit and explicit actually means.
    It's not a class constructor function. I want to ban passing the an object of the type pp to another function by reference or as a pointer.
    BTW, explicit doesn't help of any constructor / copy constructor I have right now. Plus it's not calling any constructor since it's a reference. It just passes the address of the object.
    Last edited by Elysia; 10-25-2007 at 02:25 PM.

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    I suppose you could make operator &() private to prevent taking pointers of it.

    I can't think of a way to prevent references though. Void is the only type that you can't have a reference to.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    Awesome! Now it's now possible to pass the object as a pointer by using &object.
    Still, there's no way to prevent references anyone know of?

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    You'd still need to make the constructors protected to prevent allocating on the heap. This is often done in conjunction with the factory pattern.

    Alternatively, you could make operator new() private in all it's forms (There are several).
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  9. #9
    and the hat of sweating
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    I'm not quite sure I understand?

    Do you have a particular function that you want to ban pass-by-reference for, or do you want to do it in general for every function everywhere (whether it's a function you wrote or not)?

    Is this for a class that you own & control, or for a 3rd party class?

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yes, for all functions in general, and yes it's my own class.
    I'm trying to prevent it because it's dangerous (for this class; it's not design to be passed by reference).

    I also encountered something weird:
    Code:
    ppnew<int>* pTest2 = new ppnew<int>;
    Of course it complains that it can't access private operator new, but it also complains it can't access private operator delete.
    If I make delete public or remove it, it compiles but there's no call to delete.
    Is it a safety precaution seeing as that after you allocate it, you can't delete it?

    So far, here's what I have:
    Code:
    void foo(pp<int>&);
    void foo(pp<int>*);
    
    ppnew<int> pTest; // Works, and should work
    ppnew<int>* pTest2 = new ppnew<int>; // Does not work - can't call private operator new and private operator delete, as it should be
    foo(pTest); // Works, but shouldn't! BAD!!!
    foo(&pTest); // Doesn't work, can't call private operator &
    foo(pTest2); // Works, but you can't allocate a new instance of the class
    Last edited by Elysia; 10-25-2007 at 05:29 PM.

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    >> I'm trying to prevent it because it's dangerous

    I don't see how it's dangerous. The reference count won't be incremented when you pass by reference or pass by pointer. It also won't be decremented when the function ends. So there's no danger.

  12. #12
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Daved View Post
    >> I'm trying to prevent it because it's dangerous

    I don't see how it's dangerous. The reference count won't be incremented when you pass by reference or pass by pointer. It also won't be decremented when the function ends. So there's no danger.
    Why? Because if somehow the pointer counter reaches 0 while the function is using it, it will use freed memory and crash. Since the function will never increment the ref count, it's in trouble. When might that happen? I'll be honest with you that I don't really know right now. I can't think of a scenario, but that doesn't mean it won't happen. Not to mention I find it stupid to pass a pointer by reference.
    A pointer to the memory handler is even more dangerous since it wouldn't increase the ref count either and if, say, a thread uses that pointer and the other functions don't need it anymore and get rid of it, then we have a problem.

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    Quote Originally Posted by Elysia View Post
    So far, here's what I have:
    Code:
    ppnew<int>* pTest2 = new ppnew<int>; // Does not work - can't call private operator
    What about:

    Code:
    ppnew<int>* pTest1 = new(std::nothrow) ppnew<int>; 
    ppnew<int>* pTest2 = new ppnew<int>[10];
    char pTest3a[sizeof(ppnew<int>)];
    ppnew<int>* pTest3b = new(pTest3a) ppnew<int>();
    And other cases.

    If you're going to hide new, you need to hide all its forms.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

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    >> Because if somehow the pointer counter reaches 0 while the function is using it, it will use freed memory and crash.
    It can't happen. Pretend the function is inline, then move the code from the function into the calling code. You're not calling any functions at that point. If you assume that code without function calls is safe, then code with function calls and pass by reference is safe.

    Assuming the constructor(s) increment the count, destructors decrement the count, and copying of the pointer is handled properly, there is no way there can be an issue. If you have a multi-threaded environment, then you need to put some sort of synchronization control around the decrement and delete areas, but that's not a problem for the code that uses the class.


    >> Not to mention I find it stupid to pass a pointer by reference.
    It does make sense in some situations to pass a pointer by reference, but I don't think that's what you meant. In this situation, if passing your class by reference doesn't make sense, that's fine, but don't worry about coming up with convoluted ways to prevent it when allowing it won't cause any problems.

  15. #15
    C++まいる!Cをこわせ! Elysia's Avatar
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    It seems there are dents in the armor. pTest2 and pTest3a works.
    Here are the current overloaded news:

    Code:
    	void* operator new(size_t _Count) throw();
    	void* operator new(size_t _Count, const std::nothrow_t&) throw();
    	void* operator new(std::size_t _Count, void* _Ptr) throw();
    A thought: are these "protected" new operator inherited?
    ppnew = CMemoryManagerNew: public CMemoryManager

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