C++ Vectors versus normal arrays

This is a discussion on C++ Vectors versus normal arrays within the C++ Programming forums, part of the General Programming Boards category; I've been going more seriously into standard objects of C++. The vector seems a good one - but when should ...

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    C++ Vectors versus normal arrays

    I've been going more seriously into standard objects of C++. The vector seems a good one - but when should I use it, and when should I use the normal array (the one with square brackets) ?

    Also, how can I declare a multidimensional array and vector? Up to now, I've been using only one dimension for arrays

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    always use vectors!
    i don't know of a really good reason to not use them in c++... they haven't held me back yet.

    2d vector:
    vector< vector<int > > v2d;

    2d array:
    int test[10][12];
    Last edited by simpleid; 08-09-2007 at 01:11 PM.

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    System Novice siavoshkc's Avatar
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    You can use static arrays when you know the number of items to be stored, at compile time (not at run-time). You should use dynamic arrays when this condition is not met.
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    well, with vectors you can specify a static (starting) value to immediately allocate space for what ever it is your storing... so static amount of data doesn't really mean you need c-style arrays.

    i do wonder if there's any performance loss/gain though, maybe overhead? it would only be memory though i'm guessing, most pc's these days have plenty of that though.


    edit: ah hah! from the creator of c++ [arrays are gross]
    http://www.research.att.com/~bs/bs_faq2.html#arrays

    uses vectors for his example of a simple program, because vectors rock? :-)
    http://www.research.att.com/~bs/bs_f...simple-program
    Last edited by simpleid; 08-09-2007 at 02:56 PM.

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    There can be performance benefits to using C style arrays when the size is a compile-time constant. There's nothing wrong with using vector in all cases, since those gains are often not noticable.

    Also, there is a new container that is not quite standard that can be used for static arrays (i.e. arrays with compile time constant size). That container is called array and takes the size of the array as one of its template arguments. I know it is available from boost and I believe it is part of TR1 as well. If you want a C++ alternative to static arrays then use that. For dynamic arrays use vectors.

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    With compile-time sized arrays, you may gain a little bit of performance over run-time sized ones, because the compiler can use tricks to quickly calculate a fixed multiplication. If the compiler doesn't know the size when it compiles, it obviously can't use such tricks, and must resort to proper multiplication. This is particularly "bad" if the size is a power-of-two (2, 4, 8, 16, 32 ...).

    Whether this is at all noticable depends very much on how much data is being used, and how much of the time is spent walking the array data. In most cases, it's very likely that there will be no noticable difference.

    As to usage of memory - if you don't actually "touch" the memory, it's most likely not going to actually take up any physical space in your machine.

    Say you add a global variable like this to a small application:
    Code:
    int a[10000000];  // 40MB roughly.
    Then run the application and see how much memory it uses. I think you'll find that it uses roughly the same as without such an array. Add a loop to fill the array, and memory usage goes up to around 40MB.

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    "vector is the type of sequence that should be used by default." ([C++03] §23.1.1)
    If you doubt this advice, ask yourself if you really have a compelling reason not to use the only standard container that guarantees all of the following properties. vector alone is:

    • Guaranteed to have the lowest space overhead of any container (zero bytes per object).
    • Guaranteed to have the fastest access speed to contained elements of any container.
    • Guaranteed to have inherent locality of reference, meaning that objects near each other in the container are guaranteed to be near each other in memory, which is not guaranteed by any other standard container.
    • Guaranteed to be layout-compatible with C, unlike any other standard container. (See Items 77 and Chapter 78)
    • Guaranteed to have the most flexible iterators (random access iterators) of any container.
    • Almost certain to have the fastest iterators (pointers, or classes with comparable performance that often compile away to the same speed as pointers when not in debug mode), faster than those of all other containers.
    See: http://www.ubookcase.com/book/Addiso...6lev1sec2.html

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    System Novice siavoshkc's Avatar
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    Then run the application and see how much memory it uses. I think you'll find that it uses roughly the same as without such an array. Add a loop to fill the array, and memory usage goes up to around 40MB.
    Its strange. Actually with static array it should never use more than its initial usage. Because as I know that memory is allocated on the stack. Your program should not run. Maybe its a kind of compiler optimization. Because in my compiler in Debug mode it generates an error at run-time "Stack Overflow" but in optimized mode program runs. Or maybe in optimized mode it doesn't check for overflow at run-time.
    Last edited by siavoshkc; 08-10-2007 at 10:48 PM.
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    ah hah! from the creator of c++ [arrays are gross]
    Both the problems mentioned should be fixed by std::tr1::array, which I believe was taken from boost's array, which in turn was inspired by Stroustrup's own c_array.

    uses vectors for his example of a simple program, because vectors rock? :-)
    No, because vectors are appropriate for that situation as the number of numbers to be read in is not known at compile time.

    Actually with static array it should never use more than its initial usage. Because as I know that memory is allocated on the stack. Your program should not run. Maybe its a kind of compiler optimization. Because in my compiler in Debug mode it generates an error at run-time "Stack Overflow" but in optimized mode program runs.
    I agree, it would be a compiler optimisation. The "Stack Overflow" error is probably because the statically allocated array is too large to fit on the stack, at least without this optimisation. If it could fit on the stack, there is not reason why the program would not run, even without the optimisation.
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    The "Stack Overflow" error is probably because the statically allocated array is too large to fit on the stack
    Of course. This can be resolved by changing the linker option for stack size.
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    oops (delete) :-)

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