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Thread: Tasteless mimic of operator new: Call ctor

  1. #1
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    Tasteless mimic of operator new: Call ctor

    Hello Gurus,

    I'm working on a few generic code sequences which should in some ways mimic the functionality of global operator new. I believe the code might be considered in bad taste. However, I would still be curious if there are syntaxes available for this.

    I want to mimic new in so far as it takes (or allocates) a chunk of memory and calls a ctor on it.

    My codes are below. The upper ctor call works for VS2010 but not for GCC 4.5.0. The template works for neither.

    • Can anyone help out with the syntaxes of these codes?
    • Is it even possible to call ctors directly on a chunk of memory in these ways?

    Thanks. Sincerely, Chris.

    Code:
    #include <iostream>
    
    typedef unsigned char UINT8;
    
    struct Wrapper
    {
      unsigned x;
      Wrapper(const unsigned X = 1u) : x(X) { }
    };
    
    namespace
    {
      UINT8 pool[64u];
    }
    
    template<typename T> T* alloc(void)
    {
      ((T*) pool)->T::T();
    }
    
    int main()
    {
      // Can not call constructor directly?
      // Is there a syntax for this?
      ((Wrapper*) pool)->Wrapper::Wrapper();
      const Wrapper* p1 = (Wrapper*) pool;
      std::cout << p1->x << std::endl;
    
      // Does not seem to work.
      // Wrapper does not have member function T.
      // Any syntax for this?
      const Wrapper* p2 = alloc<Wrapper>();
      std::cout << p2->x << std::endl;
    }
    You're gonna go blind staring into that box all day.

  2. #2
    Lindley is offline Elite Member Power Poster
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    Re: Tasteless mimic of operator new: Call ctor

    Use placement new.

  3. #3
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    Re: Tasteless mimic of operator new: Call ctor

    I believe placement-new is what you're looking for.

    Regards,

    Paul McKenzie

  4. #4
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    Re: Tasteless mimic of operator new: Call ctor

    Yes, thank you.

    [Edited]: Disregard previous text.
    Last edited by dude_1967; November 24th, 2010 at 02:46 PM. Reason: Post should be disregarded
    You're gonna go blind staring into that box all day.

  5. #5
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    Re: Tasteless mimic of operator new: Call ctor

    Oh... Silly me! You guys mean it's already there.

    I got it.

    Please disregard my last post.

    Thanks so much.

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

  6. #6
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    Re: Tasteless mimic of operator new: Call ctor

    Quote Originally Posted by dude_1967 View Post
    I got it.
    So just one more thing...

    Can I coerce placement new to return 0 if my custom buffer is full?

    If so, how?

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

  7. #7
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    Re: Tasteless mimic of operator new: Call ctor

    Debugging is twice as hard as writing the code in the first place.
    Therefore, if you write the code as cleverly as possible, you are, by
    definition, not smart enough to debug it.
    - Brian W. Kernighan

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  8. #8
    Lindley is offline Elite Member Power Poster
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    Re: Tasteless mimic of operator new: Call ctor

    Quote Originally Posted by dude_1967 View Post
    So just one more thing...

    Can I coerce placement new to return 0 if my custom buffer is full?

    If so, how?

    Sincerely, Chris.
    You wouldn't even try to use placement new if you didn't have memory to use it on. So the decision of whether or not the buffer is full occurs before placement new is called.

  9. #9
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    Re: Tasteless mimic of operator new: Call ctor

    Quote Originally Posted by Lindley View Post
    You wouldn't even try to use placement new if you didn't have memory to use it on. So the decision of whether or not the buffer is full occurs before placement new is called.
    May I clarify my question now that I have worked on this a while?

    If my custom buffer returns (void*) 0u, will placement new correspondingly return 0?

    For a bit of background: I am working on ultra bare-bones microcontroller embedded systems with tiny, self-managed heaps to be used in an extremely restrictive fashion, such as one-shot allocation of selected few singleton instances with static linkage. I build GCC and the CLIB without a heap or patch the compiler such that there is none. There will be no corresponding calls to delete in these designs. My stuff works quite well now that you all have steered me in the right direction on this new-thang.

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

  10. #10
    Lindley is offline Elite Member Power Poster
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    Re: Tasteless mimic of operator new: Call ctor

    If your attempt to obtain memory for an object fails, then you should not even try to call placement new because you have nowhere to place the object.

  11. #11
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    Re: Tasteless mimic of operator new: Call ctor

    Quote Originally Posted by Lindley View Post
    If your attempt to obtain memory for an object fails, then you should not even try to call placement new because you have nowhere to place the object.
    Right now my custom buffer returns zero if the call to placement new exceeds the available memory in the buffer. Perhaps I should add to my custom buffer a public member which can be queried regarding the amount of free memory available. Is this what you mean?

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

  12. #12
    Lindley is offline Elite Member Power Poster
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    Re: Tasteless mimic of operator new: Call ctor

    You seem to be thinking that the call to placement new is above the buffer allocation. I'm thinking it should be the other way around: your custom allocator decides if it has enough buffer space available, then calls placement new if so.

  13. #13
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    Re: Tasteless mimic of operator new: Call ctor

    Quote Originally Posted by Lindley View Post
    You seem to be thinking that the call to placement new is above the buffer allocation. I'm thinking it should be the other way around: your custom allocator decides if it has enough buffer space available, then calls placement new if so.
    Yes, what an astute observation. Thank you.

    I tried it that way a few times, but lost confidence because of the complexity of the solution details.

    At first, I actually did make a global allocator for objects. But I really got lost in the implementation details. I believe that you need a lot of templates or tuples to do this because you need to support ctor calls with an unknown number of ctor input parameters. This would be a template with N+1 parameters, N ctor parameters plus one additional template type for the type of the class or struct. I wrote 10 individual templates for it, so as tu support ctors with up to 10 ctor params. And it worked just fine. I tried it with a few ctors with several parameters. Everything worked fine --- OK. But the implementation details were so ugly and crude, base and low, that I was ashamed of what I had done. So I switched it to the other way around --- on purpose. Placement new gets called, custom buffer allocates and may return 0.

    Are you following this junk that I'm talking about? Or am I just totally on the wrong track?

    Could you indicate any sensible way to create a global object allocator, I mean fully generic, to be used with placement new in combination with a custom buffer?

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

  14. #14
    Lindley is offline Elite Member Power Poster
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    Re: Tasteless mimic of operator new: Call ctor

    Well, in theory, C++0x Variadic Templates are precisely what you need to solve the above problem. Unfortunately, I don't believe any current compilers yet support them.

  15. #15
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    Re: Tasteless mimic of operator new: Call ctor

    Oh. I have heard of those, variadic templates. They came up in a previous post.

    Maybe I'll go into the intricate depths of the templates and try it again.

    Maybe I can make it prettier the next time around. The first time, I had completely templated the class or struct. But I just looked at it again. In fact, I can make one allocator struct and merely template the allocator proper. So I can get away with only specializing tha allocator per-se.

    I will look at it again.

    I am happy with the solution that I have. But I still want to look at various options.

    Thanks for your help.

    Sincerely, Chris.
    You're gonna go blind staring into that box all day.

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