allocator::destroy

This is a discussion on allocator::destroy within the C++ Programming forums, part of the General Programming Boards category; I'm working on a vector-type class, but I'm having trouble with my remove function. I'm using std::allocator to allocate memory ...

  1. #1
    dra
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    allocator::destroy

    I'm working on a vector-type class, but I'm having trouble with my remove function.

    I'm using std::allocator to allocate memory for my vector, and I'm trying to use the destroy member to delete an element like this:

    Code:
    void destroy_test(iterator todelete){
     
      allocated_memory.destroy(todelete);
    
    }
    but if i try this on a vector that contains the ints 1, 2, 3, 4
    Code:
    vec.destroy_test(vec.begin());
    it still prints out 1, 2, 3, 4?

  2. #2
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    Presumably, your destroy doesn't actually remove the data from the vector itself, which leaves the data in there.

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    dra
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    But why does this happen? Shouldn't allocator::destroy call the object's destructor?

    http://msdn2.microsoft.com/en-us/library/83f353w9.aspx

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    And does the destructor remove it from the vector?

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  5. #5
    dra
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    alright so i tried using std::string and destroy seems to work on it. guess i had to find out the hard way that built-in types don't have destructors...

  6. #6
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    So you're accessing destructed strings now? That's undefined behaviour.
    All the buzzt!
    CornedBee

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  7. #7
    dra
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    No, I just used that function to illustrate my problem. I would never try something like that

  8. #8
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    I don't think you follow what I meant: When destroying something WITHIN a vector, it doesn't mean that the vector element itself is removed - just that the object the vector held in itself is destroyed. Which, as CornedBee is saying, is undefined behaviour [and it will most often actually work fine for small test-cases, but crash badly when in a more complex setup].

    To remove something from within a vector, you need to use a function like vector::erase().

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  9. #9
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    Okay, but I think you also misunderstand the purpose of my post. It wasn't to create my own erase function to use on std::vector, but instead an erase function for my own class that is supposed to mimic a vector (it was a chapter exercise). Sorry if I wasn't being clear about that in the beginning...

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