allocating structs within STL vectors

This is a discussion on allocating structs within STL vectors within the C++ Programming forums, part of the General Programming Boards category; Hi. I have a question related to memory allocation within STL vectors. I wonder if the way I am allocating ...

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    allocating structs within STL vectors

    Hi. I have a question related to memory allocation within STL vectors. I wonder if the way I am allocating memory is correct and if de-allocation is being handled.

    I am trying to use a vector of an object of my creation. There are pointers within the struct than need to be allocated. A simple example of what I'm trying to do follows:

    Code:
    struct my_struct
    {
    double *xyz;
    }
    
    vector<my_struct> my_vector;
    
    my_struct ms;
    ms.xyz = new double[xyz_size];
    
    my_vector.push_back(ms);
    Is this correct? I always thought you had to have a delete for every new. But in this case, I'm relying on the vector clean-up to deallocate the memory for double* xyz

    thanks

  2. #2
    The larch
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    No, the vector only cleans up the memory it allocates. If you store dynamically allocated objects or objects that allocate memory dynamically, then freeing that is your responsibility.

    However, why does your stuct handle memory manually? Couldn't you simply make that double* a std::vector<double> too?

    If that is for some reason impossible, consider using constructors and destructors. If that struct had a destructor that would clean up after the instance, clearing up the vector would happen automatically.
    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).

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    C++ Witch laserlight's Avatar
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    But in this case, I'm relying on the vector clean-up to deallocate the memory for double* xyz
    It does not do such a thing. It relies on my_struct to clean itself up, and my_struct just cleans up the pointers, not what they point to.

    The simple fix is to change xyz from a double* to a vector<double>. This time, the vector xyz can be relied on to deallocate the memory.
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    Quote Originally Posted by aoiOnline View Post
    Hi. I have a question related to memory allocation within STL vectors. I wonder if the way I am allocating memory is correct and if de-allocation is being handled.

    I am trying to use a vector of an object of my creation. There are pointers within the struct than need to be allocated. A simple example of what I'm trying to do follows:

    Code:
    struct my_struct
    {
    double *xyz;
    }
    
    vector<my_struct> my_vector;
    
    my_struct ms;
    ms.xyz = new double[xyz_size];
    
    my_vector.push_back(ms);
    Is this correct? I always thought you had to have a delete for every new. But in this case, I'm relying on the vector clean-up to deallocate the memory for double* xyz

    thanks
    A vector (or any STL class) has no idea of what you're doing with your class, so it's certainly not going to delete any pointer it finds. If you allocate it, you need to delete it.

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    Thanks very much guys for the instant answer. It's not impossible to use vector<double> but impractical in this case. Perhaps it's best to have a destructor as I think that would be the minimum code change.

    Thanks again.

  6. #6
    The larch
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    You'll also need a copy constructor and overloaded assignment operator. Objects in vectors need to be copiable, and copying an object managing dynamic memory needs special care.
    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).

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    Thanks. I hope I don't need an assignment operator for a double*. I'm rather sloppy when it comes to copy constructors and I try to pass variables as a reference as much as possible. Maybe it is best to use vector<double> after all.

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    C++ Witch laserlight's Avatar
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    I hope I don't need an assignment operator for a double*.
    You cannot define an assignment operator for the double*, but you should define one for my_struct if you do not turn the double* into a vector<double>.

    Maybe it is best to use vector<double> after all.
    Most likely yes. Otherwise you will be writing code for my_struct that probably duplicates the logic of a vector.
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    Quote Originally Posted by laserlight View Post
    you should define one for my_struct if you do not turn the double* into a vector<double>.
    Gotcha. Thanks for helping me out and bearing with a fledgling programmer.

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    A semi-related question about choosing between double* and vector<double>. I was under the impression that using vector<double> makes a code run slower than if double* is used. Is this true? (Using Microsoft Visual C++)

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    Quote Originally Posted by aoiOnline View Post
    A semi-related question about choosing between double* and vector<double>. I was under the impression that using vector<double> makes a code run slower than if double* is used. Is this true? (Using Microsoft Visual C++)
    Not necessarily. Read: Chapter 76. Use vector by default. Otherwise, choose an appropriate container

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    Captain Crash brewbuck's Avatar
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    The only place that xyz should be new'd is in the constructor, and it should be delete'd in the destructor. You are programming in a highly UN-object-oriented style here.

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    Quote Originally Posted by aoiOnline View Post
    A semi-related question about choosing between double* and vector<double>. I was under the impression that using vector<double> makes a code run slower than if double* is used. Is this true? (Using Microsoft Visual C++)
    How the heck could adding a layer of indirection make code run FASTER? Yeah, a vector of double* is going to take up less SPACE than a vector of double, but the doubles themselves still exist somewhere so you're using MORE memory (you need values AND pointers to the values) and making it SLOWER to access them because you have to go through a pointer.

    And you get to futz with allocating and deallocating them. On the whole, a very dumb plan.

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    Thanks for the replies.

    You are programming in a highly UN-object-oriented style here.
    I'm certain you are right. I'm not a programmer by training. I'm learning as I go.

    How the heck could adding a layer of indirection make code run FASTER? Yeah, a vector of double* is going to take up less SPACE than a vector of double, but the doubles themselves still exist somewhere so you're using MORE memory (you need values AND pointers to the values) and making it SLOWER to access them because you have to go through a pointer.
    Sorry for the confusion brewbuck. Either that or I don't understand your answer.

    I didn't mean a vector<double*> I meant using a vector<double> vs. double*

    I am writing a program that includes functions that must be lighting fast. I'm not knowledgeable in optimization techniques, so any up-front time savings help.

    Bottom line appears to be that I should use vector<double> instead of double*

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    Quote Originally Posted by aoiOnline View Post
    I am writing a program that includes functions that must be lighting fast. I'm not knowledgeable in optimization techniques, so any up-front time savings help.
    Up front time savings are a bad idea, because they can lead to hard to track bugs, particularly for the inexperienced.
    Last edited by King Mir; 12-04-2007 at 11:40 PM.
    It is too clear and so it is hard to see.
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    He could have cooked his rice much sooner.

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