Thread: dealing with array sizes

I've come across this:

Code:

 
#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;

class Chair
{
   public:
      vector<int> arrysize(int selectedArray[])
         {
            vector<int> result(2,0);        
            result[0] = sizeof(selectedArray);
            result[1] = sizeof(selectedArray[0]);
            return result;
          }
};

int main(int argc, char *argv[])
{
   Chair seat;
   int sA[] = {2,4,7,3,7};
   cout<<"sizeof(sA): "<<sizeof(sA)<<endl;
   cout<<"sizeof(sA[0]): "<<sizeof(sA[0])<<endl;
   cout<<"sizeof(sA) using arrysize(): "<<seat.arrysize(sA)[0]<<endl;
   cout<<"sizeof(sA[0]) using arrysize(): "<<seat.arrysize(sA)[1]<<endl;
   system("PAUSE");   
   return EXIT_SUCCESS;
}

Now this pops out the following results:

sizeof(sA): 20
sizeof(sA)[0]): 4
sizeof(sA) using arrysize(): 4
sizeof(sA[0]) using arrysize(): 4


My question is why is this giving different results when i get the size of the array through the function as opposed to getting the size of the array within the main loop?

I might not do the best job explaining this in terms of a low level aspect, maybe somone could back me up, but when you take the sizeof the array in the main() it actually takes all 20 bytes (4 bytes per int) as the size of the array on the stack. In the main, the array is actually sitting on the stack, it is not a pointer to data elsewhere. When you call the function, all that happens however, is a pointer to the data on the stack in the main is push'ed. If you throw something like:

Code:

std::cout << (void*) &selectedArray;

In your Chair member function, then you will see it spit out an address like: 0012FED4. Which is the address of the stack on XP SP2 for me. I think it's much higher on *nix. Um, yeah, I think that's about it.

Yes. When arrays are passed to functions, it is actually a pointer to the array which is passed. Either pass the size as a parameter, or, preferably, use std::vector.