Why isn't this program crashing? (shallow copy) - c++

Question

I am learning about copy constructor and shallow and deep copy. I was following this video: Copying and Copy Constructors in C++
The following code is copied directly from the video and demonstrates shallow copying... (at 9:30 in the video)
This code is supposed to crash after running because the deconstructor will try to deallocate the same memory twice (first for string and later for string2). The first deletion should work fine but the second one should cause the program to crash as we are trying to delete unallocated memory.
Surprisingly, this doesn't happen in my case. I complied code in my command prompt using g++ copying_and_copy_constructor.cpp and it compiled fine and ran it using a.exe. No errors.
#include<iostream>
#include<cstring>
#include<string>
using std::endl;
using std::cout;
class String
{
private:
char* m_Buffer;
unsigned int m_size;
public:
String(const char* string)
{
m_size = strlen(string);
m_Buffer = new char[m_size+1];
memcpy(m_Buffer,string,m_size);
m_Buffer[m_size] = 0;
}
~String()
{
delete [] m_Buffer;
}
friend std::ostream& operator << (std::ostream& stream, const String& string);
};
std::ostream& operator<<(std::ostream& stream, const String& string)
{
stream<< string.m_Buffer;
return stream;
}
int main()
{
String string = "My string";
String string2 = string;
cout<<string2<<endl;
cout<<string;
return 0;
}
I even tried checking in the debugger. I compiled the code using g++ -g copying_and_copy_constructor.cpp and then gdb a.exe. Here is the output of that:
GNU gdb (GDB) 7.6.1
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "mingw32".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from D:\a.exe...done.
(gdb) start
Temporary breakpoint 1 at 0x401446: file copying_and_copy_constructor.cpp, line 43.
Starting program: D:\a.exe
[New Thread 44892.0x8258]
[New Thread 44892.0x6df8]
[New Thread 44892.0xadd8]
[New Thread 44892.0x9658]
Temporary breakpoint 1, main () at copying_and_copy_constructor.cpp:43
43 String string = "My string";
(gdb) c
Continuing.
My string
My string[Inferior 1 (process 44892) exited normally]
(gdb)
The program is not being run.
(gdb) q
This line confuses me My string[Inferior 1 (process 44892) exited normally]. How is this code is exiting normally?
The person in the video is using VS-Code... How can I get the same error in the command prompt?
Crash as shown in the video:
(I don't have vs code as of now (might install it later))

Answer 1

For me (Fedora 32 x86_64) it does crash:
$ g++ -o q q.C -Wall -g;./q
My string
free(): double free detected in tcache 2
Aborted
But I definitely agree such code may look running fine. This is why there is ASAN:
$ g++ -o q q.C -Wall -g -fsanitize=address;./q
My string
=================================================================
==3388436==ERROR: AddressSanitizer: attempting double-free on 0x602000000010 in thread T0:
#0 0x7fb76f262cd7 in operator delete[](void*) (/lib64/libasan.so.6+0xb2cd7)
#1 0x4016e9 in String::~String() /home/jkratoch/t/q.C:25
#2 0x40144b in main /home/jkratoch/t/q.C:42
#3 0x7fb76ecbc041 in __libc_start_main ../csu/libc-start.c:308
#4 0x40120d in _start (/quad/home/jkratoch/t/q+0x40120d)
0x602000000010 is located 0 bytes inside of 10-byte region [0x602000000010,0x60200000001a)
freed by thread T0 here:
#0 0x7fb76f262cd7 in operator delete[](void*) (/lib64/libasan.so.6+0xb2cd7)
#1 0x4016e9 in String::~String() /home/jkratoch/t/q.C:25
#2 0x40143e in main /home/jkratoch/t/q.C:43
#3 0x7fb76ecbc041 in __libc_start_main ../csu/libc-start.c:308
previously allocated by thread T0 here:
#0 0x7fb76f2621d7 in operator new[](unsigned long) (/lib64/libasan.so.6+0xb21d7)
#1 0x4015d6 in String::String(char const*) /home/jkratoch/t/q.C:18
#2 0x4013a0 in main /home/jkratoch/t/q.C:42
#3 0x7fb76ecbc041 in __libc_start_main ../csu/libc-start.c:308
SUMMARY: AddressSanitizer: double-free (/lib64/libasan.so.6+0xb2cd7) in operator delete[](void*)
==3388436==ABORTING
In the case you do not want or even cannot recompile the program with -fsanitize=address you can use Valgrind (but it is slower and it does not detect everything):
$ g++ -o q q.C -Wall -g;valgrind ./q
==3388447== Memcheck, a memory error detector
==3388447== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3388447== Using Valgrind-3.16.0 and LibVEX; rerun with -h for copyright info
==3388447== Command: ./q
==3388447==
My string
==3388447== Invalid free() / delete / delete[] / realloc()
==3388447== at 0x483C59C: operator delete[](void*) (vg_replace_malloc.c:649)
==3388447== by 0x40139E: String::~String() (q.C:25)
==3388447== by 0x401277: main (q.C:42)
==3388447== Address 0x4db5c80 is 0 bytes inside a block of size 10 free'd
==3388447== at 0x483C59C: operator delete[](void*) (vg_replace_malloc.c:649)
==3388447== by 0x40139E: String::~String() (q.C:25)
==3388447== by 0x40126B: main (q.C:43)
==3388447== Block was alloc'd at
==3388447== at 0x483B582: operator new[](unsigned long) (vg_replace_malloc.c:431)
==3388447== by 0x401334: String::String(char const*) (q.C:18)
==3388447== by 0x40121B: main (q.C:42)
==3388447==
My string==3388447==
==3388447== HEAP SUMMARY:
==3388447== in use at exit: 0 bytes in 0 blocks
==3388447== total heap usage: 3 allocs, 4 frees, 73,738 bytes allocated
==3388447==
==3388447== All heap blocks were freed -- no leaks are possible
==3388447==
==3388447== For lists of detected and suppressed errors, rerun with: -s
==3388447== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)