The following example code demonstrates the use of uninitialized memory access and how the same can be detected using Memcheck:

#include <iostream>
using namespace std;

class MyClass {
    private:
       int x;
    public:
      MyClass( );
  void print( );
}; 

MyClass::MyClass() {
    cout << "\nMyClass constructor ..." << endl;
}

void MyClass::print( ) {
     cout << "\nValue of x is " << x << endl;
}

int main ( ) {

    MyClass obj;
    obj.print();
    return 0;

}

Let's now compile and detect the uninitialized memory access issue using Memcheck:

g++ main.cpp -g

valgrind ./a.out --track-origins=yes

==51504== Memcheck, a memory error detector
==51504== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==51504== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==51504== Command: ./a.out --track-origins=yes
==51504== 

MyClass constructor ...

==51504== Conditional jump or move depends on uninitialised value(s)
==51504== at 0x4F3CCAE: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CEDC: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F493F9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x40095D: MyClass::print() (uninitialized.cpp:19)
==51504== by 0x4009A1: main (uninitialized.cpp:26)
==51504== 
==51504== Use of uninitialised value of size 8
==51504== at 0x4F3BB13: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CCD9: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CEDC: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F493F9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x40095D: MyClass::print() (uninitialized.cpp:19)
==51504== by 0x4009A1: main (uninitialized.cpp:26)
==51504== 
==51504== Conditional jump or move depends on uninitialised value(s)
==51504== at 0x4F3BB1F: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CCD9: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CEDC: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F493F9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x40095D: MyClass::print() (uninitialized.cpp:19)
==51504== by 0x4009A1: main (uninitialized.cpp:26)
==51504== 
==51504== Conditional jump or move depends on uninitialised value(s)
==51504== at 0x4F3CD0C: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F3CEDC: 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/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x4F493F9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
==51504== by 0x40095D: MyClass::print() (uninitialized.cpp:19)
==51504== by 0x4009A1: main (uninitialized.cpp:26)
==51504== 
Value of x is -16778960
==51504== 
==51504== HEAP SUMMARY:
==51504== in use at exit: 72,704 bytes in 1 blocks
==51504== total heap usage: 2 allocs, 1 frees, 73,728 bytes allocated
==51504== 
==51504== LEAK SUMMARY:
==51504== definitely lost: 0 bytes in 0 blocks
==51504== indirectly lost: 0 bytes in 0 blocks
==51504== possibly lost: 0 bytes in 0 blocks
==51504== still reachable: 72,704 bytes in 1 blocks
==51504== suppressed: 0 bytes in 0 blocks
==51504== Rerun with --leak-check=full to see details of leaked memory
==51504== 
==51504== For counts of detected and suppressed errors, rerun with: -v
==51504== Use --track-origins=yes to see where uninitialised values come from
==51504== ERROR SUMMARY: 18 errors from 4 contexts (suppressed: 0 from 0)

The lines highlighted in bold in the preceding output clearly point to the exact line where the uninitialized variable is accessed:

==51504== by 0x40095D: MyClass::print() (uninitialized.cpp:19)
==51504== by 0x4009A1: main (uninitialized.cpp:26)

 18 void MyClass::print() {
 19 cout << "\nValue of x is " << x << endl;
 20 }

The preceding code snippet is shown for your reference; however, Valgrind will not show the code details.  The bottomline is that Valgrind precisely points to the line that accesses the uninitialized variable, which is normally difficult to detect using other means.