Retrive data using pointers to objects in c++ - c++

Question

This a menu based program to create a database of people and for performing operations on their name. After compilation, I am able to add a person successfully using the add function of Person class but when I retrieve the list of the added people using list function it shows garbage values instead of showing the entered names. It's the question no. 4(lab 4) in the below give doc.
https://drive.google.com/open?id=18cR9bgPlqM6q-kXBIcxg5Hpj04bkZMnW&authuser=0
#include<iostream>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
using namespace std;
class Person
{
const char *name;
public:
Person(const char* n)
{
name=n;
}
bool search(const char* substr)
{
const char *str=name;
while(*str!='\0')
{ int count=0;
if(*str==*substr)
{ const char *s=substr;
const char *p=str;
while(*s!='\0')
{
if(*p==*s)
{
count++;
p++;
s++;
}
else
break;
}
}
if(count==strlen(substr))
{
cout<<name<<endl;
return true;
}
str++;
}
return false;
}
void print()
{
cout<<name<<endl;
}
~Person()
{
cout << ":)";
}
friend class People;
};
class People
{
Person** array;
int length;
void prompt()
{
cout << "\n'A'-Add a person\n'L'-List all persons\n'S'-Search\n'Q'-Quit\n";
}
public:
People()
{
array = NULL;
length = 0;
}
void add()
{
string m;
cout << "Enter a Name:\n";
cin >> m;
Person s(m.c_str());
if (array == NULL)
array = (Person**)malloc(sizeof(Person*));
else
{
array=(Person**)realloc(array, length*sizeof(Person*));
}
array[length] =new Person(s.name);
array[length]->print();
++length;
}
void list()
{
cout << "\nThe names of the person in the list are:\n";
for (int i = 0; i <length; i++)
{
array[i]->print();
}
}
void search()
{
string a;
int flag = 0;
cout << "\nEnter a string to be found in the names present in the list:\n";
cin >> a;
cout << "\n The names with entered substring are:\n";
for (int i = 0; i <length; i++)
{
bool state=array[i]->search(a.c_str());
if (state)
flag = -1;
}
if (flag == 0)
cout << "\nNone of the names contains the entered substring!!!\n";
}
void menu()
{
char c = 'Y';
while (c != 'Q')
{
cout << "Choose an option(character):\n";
prompt();
cin>>c;
switch (c)
{
case 'A':add();
cout << "Name entered sucessfully!!!\n";
break;
case 'L':list();
break;
case 'S':search();
break;
case 'Q':c = 'Q';
break;
}
}
}
};
int main()
{
People All;
All.menu();
return 0;
}
I am not able to find any mistake in my implementation of add function. What could be the possible reason for malfunctioning of list function?

Answer 1

tl;dr
You store a pointer to the internal memory of string (m). That string gets destroyed at the end of add() so you have pointer to unallocated memory, which causes undefined behaviour.
possible solutions
Best would be to store a std::string instead of a const char * inside Person.
walkthrough
If you want a more detailed analyses: You store a pointer to a string that goes out of scope.
void add()
{
string m; // string is initialized and allocates memory for its content
cout << "Enter a Name:\n";
cin >> m; // read content
Person s(m.c_str()); // m.c_str() retrieves a pointer to the memory allocated by m
//this pointer is stored inside s
if (array == NULL)
{
array = (Person**)malloc(sizeof(Person*));
}
else
{
array=(Person**)realloc(array, length*sizeof(Person*));
}
array[length] = new Person(s.name); // s.name still points to the memory allocated by m
array[length]->print();
++length;
} //At the end of the function m gets destroyed and deallocates its memory
So after the function exits you still have stored the pointer to m.c_str() inside a persons name. This pointer now points to unallocated memory. This memory now may (or may not) be overwritten at any time. You get undefined behaviour and print garbage.