Question

Please run this code, and explain me why the output is like:

table[0]=3
table[1]=-858993567
table[2]=4

#include "stdafx.h"
struct First
{
    long data;
};

struct Second : public First
{
    char marker;
};

void printTable(First *table, int length)
{
    for (int i=0; i < length; ++i)
    {
        printf("table[%d]=%ld\n", i, table[i].data);
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    Second myTable[3];
    myTable[0].marker='a';
    myTable[1].marker='b';
    myTable[2].marker='c';

    myTable[0].data=3;
    myTable[1].data=4;
    myTable[2].data=5;

    printTable(myTable, 3);
    return 0;
}

Anyone can help me?

Answer 1

The function expects an array of First but you pass an array of Second. That just doesn’t work.

You can use a pointer to base class to pass a single derived object, but not arrays. The indexing operator [] needs to know the exact type of the objects to be able to calculate where they are.