Error in using vector pointer in a function

Question

I have this code, but it won't compile and i can't understand what is wrong - i guess the pointering of the vector is not correct. My idea was to collect some numbers in main() and store them in a vector and array, and then pass the memory address of them to a function, and using a pointers to print the data stored.

I came up with this when i read something about pointers which said that i should use them in order to save memory, so IMO the code below will not copy the contents of the vector and the array but use a pointer to access their location in memory - that's what i want to do.

#include <iostream>
#include <vector>
using namespace std;

void function(vector<int>* a, int *s)
{
    cout << "function starts.." << endl;
    for(int i=0;i<a->size();i++)
    {
        cout << a[i] << endl;
        cout << s[a[i]] << endl;
    }
    cout << "function ends..." << endl;
}


int main(void)
{
    vector<int> m;
    int s[102];
    for(int i=0;i<10;i++)
    {
        m.push_back(i*i);
        s[i*i] = i-2;
    }
    function(&m, &s);
    return 0;
}

I receive several errors on compiling, something is wrong.

Please tell me what's wrong with my code and how to fix it. thank you...

Answer 1

You should pass the vector by reference, not by pointer:

void function(vector<int>& a, int *s)

And then

function(m, ...);

Using [] on a pointer to a vector would certainly cause strange problems because it behaves as if a pointed to an array of std::vectors (while it actually only points to one). The vectors itself are never indexed by that. You could also use (*a)[...] to index the vector by the pointer.

Answer 2

if you insist in parsing by pointer then the correct syntax shoulld be:

void function(vector<int>* a, int *s[]) 
{ 
    cout << "function starts.." << endl; 
    for(int i=0;i<a->size();i++) 
    { 
        cout << (*a)[i] << endl; 
        cout << (*s)[(*a)[i]] << endl; 
    } 
    cout << "function ends..." << endl; 
} 

Answer 3

(corrected)

&s is in fact int(*)[102]: pointer to a pointer to an array of 102 items.

You should just say

function(&m, s);

This is because by old C legacy rule, an array is essentially a const pointer to its item with index 0. So s is already int*

Answer 4

This version works:

#include <iostream>
#include <vector>
using namespace std;

void function(const vector<int>& a, int s [102])
{
    cout << "function starts.." << endl;
    for(int i=0;i<(int)a.size();i++)
    {
        cout << a [i] << endl;
        cout << s[a [i]] << endl;
    }
    cout << "function ends..." << endl;
}


int main(void)
{
    vector<int> m;
    int s[102];
    for(int i=0;i<10;i++)
    {
        m.push_back(i*i);
        s[i*i] = i-2;
    }
    function(m, s);
    return 0;
}

Answer 5

First of all in the main program s is a pointer to an int, while m is a vector. Thus the function call should be as follows:

function(&m, s);

Secondly in the function a is a pointer to a vector, so should be indexed as follows: (*a)[i].

However you should really be using const references to pass your vector around:

void function(const vector& a, int *s) { .. cout << a[i] << endl; .. }

And call it like:

function(m, s);