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Jul 28th, 2009

Finding the length of an int pointer array

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I'm working on a little code snippet and my c++ seems a bit rusty. How do i go about getting the length of an array? My eclipse is giving me weird values when it should be segfaulting, so I'm trying to figure out whats wrong. Thanks in advance.

edit: forgot to say what i meant by an int array.

int *x = new int[8];

Last edited by DaBunBun; Jul 28th, 2009 at 2:17 pm.

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DaBunBun

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Jul 28th, 2009

Re: Finding the length of an int pointer array

I always advocate using an std::vector when possible.. can you do this:

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std::vector<int*> yourVar(8);
std::vector<int*> yourVar(8);

daviddoria

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Jul 28th, 2009

Re: Finding the length of an int pointer array

Are you saying to use a different variable type?
Or are you saying to type cast it?

If its the first, I want to get the length of a pointer thats being passed.

Edit: Forgot to say, thanks for the help. Normally I'd use a vector like that, except in this case I'm being passed an in pointer.

Last edited by DaBunBun; Jul 28th, 2009 at 2:30 pm.

DaBunBun

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Re: Finding the length of an int pointer array

Click to Expand / Collapse Quote originally posted by DaBunBun ...

Are you saying to use a different variable type?
Or are you saying to type cast it?

If its the first, I want to get the length of a pointer thats being passed.

Edit: Forgot to say, thanks for the help. Normally I'd use a vector like that, except in this case I'm being passed an in pointer.

I'm pretty sure it's impossible to get the length of an array from the address of the base pointer. You the programmer are responsible for keeping track of the length. That's why when you pass an array to a function, you generally pass the length as well. Regarding "length of a pointer", I assume you are referring to "number of elements in the array"? Regardless, I know of no way to get the 8 from x in your original example. Store 8 in a separate variable.

VernonDozier

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Jul 28th, 2009

Re: Finding the length of an int pointer array

Yeah, I didn't think so either, but I just wanted to check. I think there might be a way if you know what your being passed and using sizeof(), but thats not what I was looking for. Thanks for your help.

DaBunBun

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Re: Finding the length of an int pointer array

You always have to pay attention when you create a vector of pointers, consider this:

C++ Syntax (Toggle Plain Text)
#include <iostream>
#include <vector>
using namespace std;
 
int main()
{
    vector<int*> myvec(5);
 
    if( 5 > 3 )
    {
        int a = 35;
        myvec[0] = &a;
        cout << *myvec[0] << endl;
    }
 
    cout << *myvec[0] << endl; // not guaranteed to print 35 on the screen
 
    return 0;
}
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int*> myvec(5);

    if( 5 > 3 )
    {
        int a = 35;
        myvec[0] = &a;
        cout << *myvec[0] << endl;
    }

    cout << *myvec[0] << endl; // not guaranteed to print 35 on the screen
	
    return 0;
}

When variable a goes out-of-scope, then the first pointer in vector myvec (myvec[0]) will point to some memory location which doesn't belong to your program anymore.

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