Question

I have this very simple c++ program. It’s been years that I did some C++ so I thought I might give it a spin one more time. But I am amazed by the output I am getting. It was supposed to be a simple program and it’s causing me some headaches already.

//2.cpp
//This program asks for the radius of the circle and
//prints the area of that circle 

#include <iostream>
#include <stdlib.h>

int* area(char* radius[], int size)
{
        int* pointer;
        int areas[size];
        pointer = areas;
        for(int i = 0; i < size; i++)
        {
                areas[i] = 3.1416*atoi(radius[i])*atoi(radius[i]);
        }
        return pointer;
}
void print(char* radius[], int* area1, int size)
{
        std::cout<<area1[2]<<std::endl; //This prints fine
        for(int i = 0; i < size; i++)
        {
                std::cout << area1[i]; //This doesn't
                std::cout << "Area for " << radius[i] << " is: " << area1[i] << std::endl;
        }
}

int main(int argc, char* argv[])
{
        if(argc > 1)
        {
                print(&argv[1],area(&argv[1],argc-1),argc-1);
        }
        else
        {
            //Please ignore this
        }
    return 0;
}

Input

./a.out 1 4 2 7 8

Output:-

12
134520896
Area for 1 is: 134520896
10
Area for 4 is: 10
-1217419175
Area for 2 is: -1217419175
-1217056780
Area for 7 is: -1217056780
-1217056780
Area for 8 is: -1217056780

Answer 1

Your areas array has automatic storage duration and goes out of scope once area returns; then dereferencing pointer is undefined behavior. Please use C++ idioms like std::string and std::vector instead of C pointers.

Here is an improved (but still not optimal) version of your code:

#include <iostream>
#include <vector>
#include <string>
#include <cstdlib>
#include <cstddef>

std::vector<double> area(const std::vector<double>& radius)
{
  std::vector<double> areas(radius.size());
  for (std::size_t i = 0; i < radius.size(); i++) {
    areas[i] = 3.1416 * radius[i] * radius[i];
  }
  return areas;
}

void print(const std::vector<double>& radius, const std::vector<double>& area)
{
  for (std::size_t i = 0; i < radius.size(); i++) {
    std::cout << area[i]; //This doesn't
    std::cout << "Area for " << radius[i] << " is: " << area[i] << std::endl;
  }
}

int main(int argc, char* argv[])
{
  if (argc > 1) {
    std::vector<double> radii;
    radii.reserve(argc - 1);
    for (int i = 1; i < argc; ++i) {
      radii.push_back(std::atof(argv[i]));
    }
    print(radii, area(radii));
  }
}

Unfortunately compilers often cannot detect such errors. Use a tool like Valgrind to find them. For example, running your original code through Valgrind produces many errors for me:

$ valgrind ./a.out 1 4 2 7 8
==18488== Memcheck, a memory error detector
==18488== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==18488== Using Valgrind-3.8.0 and LibVEX; rerun with -h for copyright info
==18488== Command: ./a.out 1 4 2 7 8
==18488== 
==18488== Conditional jump or move depends on uninitialised value(s)
==18488==    at 0x4EC5D16: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x4EC5F4C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x4EC8E45: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x400A01: print(char**, int*, int) (in /tmp/a.out)
==18488==    by 0x400B0E: main (in /tmp/a.out)
==18488== 
==18488== Use of uninitialised value of size 8
==18488==    at 0x4EBB133: ??? (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x4EC5D37: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x4EC5F4C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x4EC8E45: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/libstdc++.so.6.0.17)
==18488==    by 0x400A01: print(char**, int*, int) (in /tmp/a.out)
==18488==    by 0x400B0E: main (in /tmp/a.out)
[…]

while my version produces no errors.