Pointers in C++

Hello experts.

void aaa(bbb* aVar)
{
      bbb myVar;

      myVar = aVar[1]; //compiles
      //myVar = *aVar[1]; //does not compile

}

This is my thinking process:

a. bbb * aVar

aVar is a pointer to address of bbb.

We can treat aVar as an array where
aVar[0] points to first address of bbb
aVar[1] points to second address of bbb, and so on.

So...

bbb = aVar[1] should make bbb equal to address of second
element of aVar

bbb = *aVar[1] should make bbb equal to value of second
element of aVar

Where do I get it wrong?

Thank you

panJames

panJames

Asked: 2011-03-01

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1 Solution

LVL 75

käµfm³d 👽Commented: 2011-03-01

>> aVar is a pointer to address of bbb.

aVar is a pointer to the location of an object of type bbb. The indexing operation ( [n] ) is equivalent to saying

*(somePtr + n)

What you are trying to do with this line:

myVar = *aVar[1];

is dereference whatever is found in the first slot of the array aVar.

LVL 75

käµfm³d 👽Commented: 2011-03-01

>> dereference whatever is found in the first slot of the array aVar.

Using the aforementioned equivalence, what you attempting to do is:

**(aVar + 1)

panJamesAuthor Commented: 2011-03-01

//
What you are trying to do with this line:

myVar = *aVar[1];

is dereference whatever is found in the first slot of the array aVar.
//

why is it illegal?

panJames

LVL 6

MistralolCommented: 2011-03-01

Thats kinda of true but what you are really doing is this

**(avar + sizeof(bbb));

void aaa(bbb* aVar)
{
bbb myVar;

myVar = aVar[1]; //compiles
//myVar = *aVar[1]; //does not compile

}

The first works because myVar is taking a copy of the data in aVar[1] from the array slot 1.
So its the same as memcpy(&myVar, &aVar[1], sizeof(myVar));

LVL 75

käµfm³d 👽Commented: 2011-03-01

My C-Fu is not that strong, but IIRC, unless aVar is a pointer to an array of pointers, array of arrays, or a pointer to a pointer, you will get an error.

LVL 75

käµfm³d 👽Commented: 2011-03-01

*with the above syntax = )

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