What will happen if someone tries to access freed memory?

[rajhansk]

Write your question here.

  
#include <iostream>

using namespace std;

int main()
{
    int* pt=new int;

    *pt= 1000;

    cout<<"The value of pt is "<<*pt<<endl;
    cout<<"The  address of pt is "<<pt<<endl;

    delete pt;

    cout<<*pt;

    return 0;
}

[rajhansk]

is it a memory leak ??

[AbstractionAnon]

What will happen if someone tries to access freed memory?

It's called "undefined behavior".

If you're running a debug build, you will probably get a trap (implementation specific). If you're running a release build, you read or write whatever is in that memory location, which might have been assigned to something else.

is it a memory leak ??

No. You allocate the pointer at line 8 and release it at line 15. No memory leak.

[E0p9LyTq]

What will happen if someone tries to access freed memory?

What exactly will happen depends on the compiler.

#include <iostream>

int main ()
{
   short* short_ptr = new short{12};

   std::cout << short_ptr << ",\t" << *short_ptr << "\n\n";

   delete short_ptr;

   long* long_ptr = new long{123456};   

   std::cout << short_ptr << ",\t" << *short_ptr << "\n";
   std::cout << long_ptr << ",\t" << *long_ptr << "\n\n";

   *short_ptr = 125; // uh-oh!

   std::cout << short_ptr << ",\t" << *short_ptr << "\n";
   std::cout << long_ptr << ",\t" << *long_ptr << "\n";
}

The example above compiles in both TDM GCC 4.9.2 and MS Visual C++ 2015 Community. Differences occur when running the resulting executable.

TDM GCC produces the following output:

0x182510,       12

0x182510,       -7616
0x182510,       123456

0x182510,       125
0x182510,       65661

MSVC++ produces the following output:

0x182510, 12

And then crashes with a "read access violation." Writing to a deleted pointer would also crash, *short_ptr = 125;, with a "write access violation" if the program had not already terminated.

One thing to note is that new assigns memory addresses differently between TDM GCC and MSVC++. MSVC++ may not assign the same memory location to a new pointer as a previously deleted pointer:

#include <iostream>

int main ()
{
   short* short_ptr = new short{12};

   std::cout << short_ptr << ",\t" << *short_ptr << "\n\n";

   delete short_ptr;

   long* long_ptr = new long{123456};   

   std::cout << long_ptr << ",\t" << *long_ptr << "\n";
}

006E2C28,       12

006E2BF8,       123456

If you are lucky trying to access a deleted pointer, read or write, will crash the program immediately. Corrupting memory while continuing to run, as TDM GCC does, is hard to debug.