after delete ...

This is a discussion on after delete ... within the C++ Programming forums, part of the General Programming Boards category; code like: Code: int* e = new int; delete e; *e = 3; // why no error? cout << *e ...

  1. #1
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    after delete ...

    code like:

    Code:
    int* e = new int;
    
    delete e;
    
    *e = 3; // why no error?
    
    cout << *e << endl; //why no error?

  2. #2
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    The compiler has no way of knowing that e is no longer a valid pointer and so dosen't generate a compiler error. The memory pointed to by e is possibly unchanged because nothing else has used it yet howerver this memory is not marked as allocated and so could be reused at any time which would mean you are overwriting some memory.
    After delete you should set the pointer to a NULL pointer to prevent these errors.
    Last edited by Quantum1024; 11-21-2005 at 06:12 AM.

  3. #3
    Cat without Hat CornedBee's Avatar
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    This is part of the reason why pointers are called unsafe: they just don't know that they're invalid and might or might not fail. But don't forget one of the many Laws of Computer Science (I think Bjarne Stroustrup first formulated this one): "Undefined behaviour means it works while writing it, works while testing it, and crashes for your most important customer."
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    I thought it was "blows up".
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  5. #5
    aoeuhtns
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    Quote Originally Posted by Quantum1024
    After delete you should set the pointer to a NULL pointer to prevent these errors.
    That doesn't work.

    Code:
    x = new int;
    *x = 5;
    y = x;
    
    delete x;
    x = NULL;
    
    *y = 7;
    There are 10 types of people in this world, those who cringed when reading the beginning of this sentence and those who salivated to how superior they are for understanding something as simple as binary.

  6. #6
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    because y conains the original value of x. when you change x to NULL that doesn't change the value of y -- the value of y remains unchanged. y is NOT a pointer to x but a copy of x.

    try this -- note the double asterisks, in this example y is a pointer to x, so when you change x you will also change the value to which y points.
    Code:
    x = new int;
    *x = 5;
    int **y = &x;
    
    delete x;
    x = NULL;
    
    **y = 7;
    Last edited by Ancient Dragon; 11-21-2005 at 11:33 AM.

  7. #7
    Frequently Quite Prolix dwks's Avatar
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    Quote Originally Posted by Ancient Dragon
    Well duuuh! is ain't suppose to work!
    I like that one.

    And "proper feedback"? Well, one of my compilers prints "NULL pointer assignment" but another one lets it pass.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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  8. #8
    Cat without Hat CornedBee's Avatar
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    It's about runtime feedback.

    Compare:
    Code:
    int *p = new int;
    *p = 5;
    delete p;
    *p = 6;
    -> will probably go undetected.

    Code:
    int *p = new int;
    *p = 5;
    delete p;
    p = 0;
    *p = 6;
    -> will crash.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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