Re: something unclear about "sizeof()" and "free()"

Alan McKenney <>
Sun, 4 Oct 2009 11:21:25 CST
On Oct 3, 1:53 pm, Paul Bibbings <> wrote:

     a pointer is a address;

Pointers can be difficult to understand initially when you are coming


them for the first time. To begin to think about them correctly, however,


should change this to:

        'a pointer "stores" an address'

    Actually, this comes up with almost any type
    -- people who talk about C (and C++) routinely
    use the same words to describe the _variable_
    ("storage location") and the value in it.
    This confuses nobody but beginners and those
    who try to explain the difference between
    "lvalue" and "rvalue."

Then we get a better sense of what is going on, which is something like


     | p |
     | 0xf51cc8 |
                  -------------- ------------
        0xf51cc8 | 0 | 1 | 2 | ... | 98 | 99 |
                  -------------- ------------

In this way it is easier to remember that the pointer p is not the same

thing as

the thing it points to.

    That is, a pointer [value] is a number
    whose value magically refers to some bit
    of storage. Think of it as a tag from the
    left luggage room -- you hand the clerk
    the tag (="dereference the pointer") and
    he/she hands you your suitcase. Obviously,
    the tag is not the same as your suitcase, and
    its size has nothing to do with the size
    of your suitcase, either.

    When you have a "malloc"ed variable or arry,
    however, the address isn't just a reference
    to your bag, so to speak. The malloc system
    "keeps a dossier" on the memory you requested,
    and it uses the address it gave you to look
    up that information. So the pointer value
    isn't just the address of some memory, it's
    also, in effect, a database key.

    This gets to one of my big beefs with
    C/C++ pointers: they are really low-level
    concepts, living fossils from the days
    when C was basically a high-level
    assembler. C uses a "machine address"
    as a "tag" for a lot of different things:

    1. a "tag" for a scalar variable (storage)
        In that case, about all you can do with
        the pointer value is dereference it
        (apply "*"). You can try to "free" it,
        but it's Undefined Behavior.

    2. a "tag" for an array.
        In that case, you can pretend it's a pointer
        to a scalar -- the first element of the array
        -- but you can also index it: "P[i]", assuming
        the value of "i" isn't negative or greater
        than the size of the array.
        In C, it's also treated as a special case
        of the next usage:

    3. a "tag" for an element of an array.
        You can still treat it as a "tag" to
        a scalar, but, if you know where in
        the array you are, you can add or subtract
        an integer (the number of elements to shift),
        assuming of course that you don't end
        up shifting beyond the beginning or end.

        BTW, there are computer architectures which
        use different bit representations for #1, #2,
        and #3.

    4. A "handle" to a dynamically allocated
        scalar. This is like #1, but you can
        feed it to "free" or "delete", depending
        upon whether you got the value from
        "malloc" or "new".

    5. A "handle" to a dynamically allocated array.
        This is like #2, but you can feed it to
        "free" or "delete []" {note the brackets!)
        Note that this is _not_ like #3. You can
        only give to "free" or "delete[]" the exact
        value you got from "malloc" or "new[]", and,
        in the case of delete[], the value also has
        to be identified as having the correct type.

    The thing is, in C, you can't tell from the type
    or the value which of these case you have, any
    more than if you look at 32 bits of memory which
    contain 0x00002020 you can tell whether this is
    the string value " " on a little-endian machine
    or the integer value 8224.

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