The following example code demonstrates memory access to the already released memory locations:

#include <iostream>
using namespace std;

int main( ) {

    int *ptr = new int();

    *ptr = 100;

    cout << "\nValue stored at pointer location is " << *ptr << endl;

    delete ptr;

    *ptr = 200;
    return 0;
}

Let's compile the preceding program and learn how Valgrind reports the illegal memory access that attempts to access an already released memory location:

==118316== Memcheck, a memory error detector
==118316== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==118316== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==118316== Command: ./a.out
==118316== 

Value stored at pointer location is 100
==118316== Invalid write of size 4
==118316== at 0x400989: main (illegalaccess_to_released_memory.cpp:14)
==118316== Address 0x5ab6c80 is 0 bytes inside a block of size 4 free'd
==118316== at 0x4C2F24B: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==118316== by 0x400984: main (illegalaccess_to_released_memory.cpp:12)
==118316== Block was alloc'd at
==118316== at 0x4C2E0EF: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==118316== by 0x400938: main (illegalaccess_to_released_memory.cpp:6)
==118316== 
==118316== 
==118316== HEAP SUMMARY:
==118316== in use at exit: 72,704 bytes in 1 blocks
==118316== total heap usage: 3 allocs, 2 frees, 73,732 bytes allocated
==118316== 
==118316== LEAK SUMMARY:
==118316== definitely lost: 0 bytes in 0 blocks
==118316== indirectly lost: 0 bytes in 0 blocks
==118316== possibly lost: 0 bytes in 0 blocks
==118316== still reachable: 72,704 bytes in 1 blocks
==118316== suppressed: 0 bytes in 0 blocks
==118316== Rerun with --leak-check=full to see details of leaked memory
==118316== 
==118316== For counts of detected and suppressed errors, rerun with: -v
==118316== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Valgrind precisely points to the line number (14) that attempts to access the memory location that got released at line number 12.