CODEWITHSUNDEEP
c++memory-leakspolymorphism
Q_A
Q_A

Reputation: 451

c++ - Memory leak and polymorphism

I've been trying to wrap my head around polymorphism in C++ From what I understand, it goes like this

class Base {
 //...
 public:
  virtual int Foo() {...} = 0;
};
//...
class Derived: public Base { //Could be protected or private as well
 //...
 public:
  int Foo() {...}
};

I also know that when we have an array of dynamically allocated objects, we must call delete [] arr at the end of the program after we delete each individual entry of the array.

So when I run the following program, I'm not sure why I get a memory leak

#include <iostream>
using namespace std;

class Number {
 public:
  ~Number() {
   cout << "Expression deleted" << endl;
  }
 virtual void print() = 0;
};

class Int: public Number {
 private:
  int num;
 public:
  Int(int n) {
   num = n;
  }
  void print() {
   cout << "Num: " << num << endl;
  }
  ~Int() {
   cout << "Number deleted" << endl;
  }
};

int main() {
 Number *arr[10];
 arr[0] = new Int(1);
 arr[1] = new Int(2);
 arr[2] = new Int(3);

 arr[0]->print();
 arr[1]->print();
 arr[2]->print();

 delete arr[0];
 delete arr[1];
 delete arr[2];
 delete [] arr;
}

It first gives me a warning when compiling

poly.cc: In function 'int main()':
poly.cc:37:12: warning: deleting array 'Number* arr [10]' [enabled by default]

Then when I run it, it gives me this 

Num: 1
Num: 2
Num: 3
Expression deleted
Expression deleted
Expression deleted
*** glibc detected *** ./a.out: munmap_chunk(): invalid pointer: 0x00007fff785732d0 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7db26)[0x7f9274641b26]
./a.out[0x400b07]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed)[0x7f92745e576d]
./a.out[0x400959]
======= Memory map: ========
00400000-00401000 r-xp 00000000 00:32 43144247                           
00601000-00602000 r--p 00001000 00:32 43144247                           
00602000-00603000 rw-p 00002000 00:32 43144247                           
01b5f000-01b91000 rw-p 00000000 00:00 0                                  [heap]
7f92740b1000-7f92740c7000 r-xp 00000000 fc:00 50593804                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7f92740c7000-7f92742c6000 ---p 00016000 fc:00 50593804                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7f92742c6000-7f92742c7000 r--p 00015000 fc:00 50593804                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7f92742c7000-7f92742c8000 rw-p 00016000 fc:00 50593804                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7f92742c8000-7f92743c3000 r-xp 00000000 fc:00 50602224                   /lib/x86_64-linux-gnu/libm-2.15.so
7f92743c3000-7f92745c2000 ---p 000fb000 fc:00 50602224                   /lib/x86_64-linux-gnu/libm-2.15.so
7f92745c2000-7f92745c3000 r--p 000fa000 fc:00 50602224                   /lib/x86_64-linux-gnu/libm-2.15.so
7f92745c3000-7f92745c4000 rw-p 000fb000 fc:00 50602224                   /lib/x86_64-linux-gnu/libm-2.15.so
7f92745c4000-7f9274778000 r-xp 00000000 fc:00 50602238                   /lib/x86_64-linux-gnu/libc-2.15.so
7f9274778000-7f9274977000 ---p 001b4000 fc:00 50602238                   /lib/x86_64-linux-gnu/libc-2.15.so
7f9274977000-7f927497b000 r--p 001b3000 fc:00 50602238                   /lib/x86_64-linux-gnu/libc-2.15.so
7f927497b000-7f927497d000 rw-p 001b7000 fc:00 50602238                   /lib/x86_64-linux-gnu/libc-2.15.so
7f927497d000-7f9274982000 rw-p 00000000 00:00 0
7f9274982000-7f9274a84000 r-xp 00000000 fc:00 41962840                   /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f9274a84000-7f9274c83000 ---p 00102000 fc:00 41962840                   /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f9274c83000-7f9274c8b000 r--p 00101000 fc:00 41962840                   /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f9274c8b000-7f9274c8d000 rw-p 00109000 fc:00 41962840                   /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f9274c8d000-7f9274c90000 rw-p 00000000 00:00 0
7f9274c90000-7f9274cb2000 r-xp 00000000 fc:00 50602228                   /lib/x86_64-linux-gnu/ld-2.15.so
7f9274e84000-7f9274e89000 rw-p 00000000 00:00 0
7f9274eae000-7f9274eb2000 rw-p 00000000 00:00 0
7f9274eb2000-7f9274eb3000 r--p 00022000 fc:00 50602228                   /lib/x86_64-linux-gnu/ld-2.15.so
7f9274eb3000-7f9274eb5000 rw-p 00023000 fc:00 50602228                   /lib/x86_64-linux-gnu/ld-2.15.so
7fff78553000-7fff78574000 rw-p 00000000 00:00 0                          [stack]
7fff785c7000-7fff785c9000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted

It seems that the objects are being added and deleted successfully. I can't see what's wrong.

Upvotes: 2

Views: 481

Answers (2)

SergeyA
SergeyA

Reputation: 62613

Because for every polymoprhic class you need to define a virtual destructor. Otherwise deleting a derived class through a pointer to base class will not call derived class destructor.

Upvotes: 1

Barry
Barry

Reputation: 303537

There are two problems here. First, this is invalid:

delete [] arr;

arr wasn't new[]-ed so it doesn't need to be delete[]-ed. arr is just on the stack. That's why your compiler gave you the warning! (clang straight up errors). Good rule of thumb is to not ignore compiler warnings.

A second problem is here:

~Number() {
    cout << "Expression deleted" << endl;
}

When you delete a Number, ~Number() will be executed and that memory will be freed up... but not ~Int(). You need to make the destructor virtual too if you're going to be deleting the base class pointers. This should've led to a warning as well, something like:

main.cpp: In function 'int main()':
main.cpp:37:14: warning: deleting object of abstract class type 'Number' which has non-virtual destructor will cause undefined behaviour [-Wdelete-non-virtual-dtor]
  delete arr[0];
              ^

Upvotes: 6

Related Questions