Difference between straight and dynamic allocation?

This is a discussion on Difference between straight and dynamic allocation? within the C++ Programming forums, part of the General Programming Boards category; I am curious about the difference between straight allocation: class_name object_name(arguements); and dynamic allocation: class_name *pointer_name = new class_name(arguements); Is ...

  1. #1
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    Difference between straight and dynamic allocation?

    I am curious about the difference between straight allocation:

    class_name object_name(arguements);

    and dynamic allocation:

    class_name *pointer_name = new class_name(arguements);

    Is it because straight allocation gives you a 'this' pointer and dynamic gives you a regular pointer? Or is that explanation too simple?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    The "straight" allocation, as you call it, means that you are passing the job of dealing with the variable off to the compiler. The compiler allocates it itself when you declare it, and the compiler makes it go away by itself when it goes out of scope.

    Something that is dynamically allocated is your responsibility -- you have to decide when you're done with it and delete it yourself.

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    The difference is mainly in the lifetime of the object. A local object (inside a function) will only live for the duration of that function. A dynamically allocated object will exist until you free it (with delete). And of course, it is possible (in most environments) to have global objects, which exist from "before main" until "after main".

    There is a "this" pointer for ALL objects. It points to the object itself, no matter when and how it was created.

    For many things, you do not need dynamic allocation, but there are certainly cases where it is needed.

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    The general rule is to use "straight allocation" or local variables and declare them in the smallest scope in which they need to be used.

    Then you use dynamic allocation only in certain other circumstances. The most common of those circumstances is when you want to create an object in a specific scope but you want it to live longer than that scope.

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Another common use is to avoid unnecessary copying of data (if you have two structs pointing to the same data, it's more efficient to keep a pointer to it).
    Of course, in C++, it's also relative easy and safe to do that with the help of smart pointers.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Frequently Quite Prolix dwks's Avatar
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    "Straight" allocation, as you call it, uses memory from the stack. Dynamic memory allocation uses memory from the heap.

    The stack is more convenient, being automatically freed and so on. [*Elysia points at auto pointers*] The heap is usually much larger, however, so you might also want to use dynamic memory allocation for very large objects.
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    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by dwks View Post
    [*Elysia points at auto pointers*]
    No, actually I was hinting at a shared pointer, eg std::tr1::shared_ptr or boost::shared_ptr.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    >> Another common use is to avoid unnecessary copying of data (if you have two structs pointing to the same data, it's more efficient to keep a pointer to it).

    That doesn't seem to be a use for dynamic allocation over statically allocated objects. You can have pointers to local objects as well.

    Using dynamic allocation to avoid copying is basically just a specific case of using it to change the lifetime of an object.

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    C++まいる!Cをこわせ! Elysia's Avatar
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    I suppose your theory is more plausible. It doesn't need to be dynamically allocated to use pointers, after all (which you do point out).
    But another very common use is (which I remember now) to allocate a lot of data. The stack is limited to about 1 MB. The heap is limited to the computer RAM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    That is another case, yes. I don't consider it quite as common because in most cases you will be using another class that does that for you (e.g. vector).

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    Thank you for very clear explanations!

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