Vector doesn't init this way....

This is a discussion on Vector doesn't init this way.... within the C++ Programming forums, part of the General Programming Boards category; I was dynamically creating a struct using new, this struct had a vector in it. The vector worked fine. Now ...

  1. #1
    Unregistered User Yarin's Avatar
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    Vector doesn't init this way....

    I was dynamically creating a struct using new, this struct had a vector in it. The vector worked fine. Now I'm dynamically creating the struct by using GlobalAlloc, but the vector doesn't init like it's suppose to, now, I know why this is the case, but I don't know how to manually init vectors. Is it even possible? What do I do at this point?
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    Frequently Quite Prolix dwks's Avatar
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    Could you perhaps call the constructor directly?

    [edit] It seems that you can call a destructor directly, if you really wanted to, but I can't see how you could call a constructor directly. Perhaps you could use new for that part of the structure? e.g.
    Code:
    struct data {
        vector<int> *v;
        int x, y, z;
        /* ... */
    };
    data x = GlobalAlloc();  // I don't know how to use this function
    x->v = new vector<int>;
    [/edit]
    Last edited by dwks; 07-09-2008 at 03:46 PM.
    dwk

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  3. #3
    The larch
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    Perhaps you need to call the constructor using placement new?

    If I'm not mistaken GlobalAlloc doesn't call the constructor of the struct (even if it is generated by the compiler it would call the constructor of the vector). You'll probably need to treat its return value as raw memory.
    I might be wrong.

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    Frequently Quite Prolix dwks's Avatar
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    http://www.parashift.com/c++-faq-lit....html#faq-10.3
    BTW do NOT try to achieve this via placement new. Some people think they can say new(this) Foo(x, int(x)+7) within the body of Foo::Foo(char). However that is bad, bad, bad. Please don't write me and tell me that it seems to work on your particular version of your particular compiler; it's bad. Constructors do a bunch of little magical things behind the scenes, but that bad technique steps on those partially constructed bits. Just say no.
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  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yes, something along the lines of:

    Code:
    struct data {
        vector<int> *v;
        int x, y, z;
        /* ... */
    };
    data x = GlobalAlloc();  // I don't know how to use this function
    new (x) data; // Placement new
    Should initialize it properly before you start venturing further.
    To my knowledge, only new calls constructors.
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  6. #6
    Kernel hacker
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    Why do you need to use GlobalAlloc like that? Would it make more sense to override the operator new() for that class, and inside that call GlobalAlloc?

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  7. #7
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Yarin View Post
    Now I'm dynamically creating the struct by using GlobalAlloc, but the vector doesn't init like it's suppose to, now, I know why this is the case, but I don't know how to manually init vectors.
    Well I can't say I haven't needed to do something like that myself before, but I do need to ask why you would use GlobalAlloc instead of new in this instance. You won't be able to just pass the whole thing to something else that uses GlobalFree on it anyway, since that would leak the vector's contents. You can't marshall it via COM etc. You really must seriously question yourself whether it is the right thing to do. Please explain.
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  8. #8
    Cat without Hat CornedBee's Avatar
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    Quote Originally Posted by Elysia View Post
    Yes, something along the lines of:

    Code:
    struct data {
        vector<int> *v;
        int x, y, z;
        /* ... */
    };
    data x = GlobalAlloc();  // I don't know how to use this function
    new (x) data; // Placement new
    Should initialize it properly before you start venturing further.
    To my knowledge, only new calls constructors.
    More precisely, it should be
    Code:
    void *mem = GlobalAlloc(...);
    data *x = new (mem) data;
    There are a few reasons for that.
    One, the return value of GlobalAlloc points to uninitialized memory, not a data object, so you shouldn't pretend it does.
    Two, you need to do it that way, since placement new isn't actually guaranteed to return the same address it was passed. That is, the single-object variant is, but the array variant is not. For consistency, make array and single-object placement new look the same.
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  9. #9
    Unregistered User Yarin's Avatar
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    I am doing this because I want my c++ code to be manageable through c, and c doesn't support the new and delete ops. And I read that you should never free memory using an operator/function that aren't directly paired. (I.E. don't free malloc with delete etc.)
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  10. #10
    The larch
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    So you can use std::vector, but you cannot use new/delete?
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  11. #11
    Unregistered User Yarin's Avatar
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    I got the solution. Now I'm just having the C code call imported C++ to do that for me (the vector was created in the imported C++ anyway)
    A class that doesn't overload all operators just isn't finished yet. -- SmugCeePlusPlusWeenie
    A year spent in artificial intelligence is enough to make one believe in God. -- Alan J. Perlis

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