Never forget to delete an array of object created in heap using delete[].
If you forget to do so its really hard to debug especially if you are doing this one in some function some where is your source code and execution hits this delete on some particular condition
As usual let’s begin with example
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class Test
{
public:
Test()
{
std::cout << "Ctor called\n";
}
~Test()
{
std::cout << "Dtor called\n";
}
};
int main()
{
Test* p = new Test[2]; //(1)
delete p; //(2)
return 0;
}
Here in this above example, Test* p is a pointer pointing to an array of Test objects. In next line we can see delete p is used. When we try to compile this source, compiler does not give an error rather it thrown some warnings. If we ignore this warning then we have a pay it for later.
Let’s compile it
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bhavith@bhavith:~$ g++ main.cpp
main.cpp: In function ‘int main()’:
main.cpp:21:12: warning: ‘void operator delete(void*, std::size_t)’ called on pointer ‘<unknown>’ with nonzero offset 8 [-Wfree-nonheap-object]
21 | delete p;
| ^
main.cpp:19:25: note: returned from ‘void* operator new [](std::size_t)’
19 | Test* p = new Test[2];
From the above compiler message we can observer that there is some function like void* operator new[](std::size_t) which we are calling on (1) i.e. new Test[2] that means we are calling an operator [] with size. Then this function returning an pointer i.e. void* and we saving to variable p
The second warning compiler is complaining is, there is a function void operator delete(void*, std::size_t) and we are calling it which we are not suppose to call.
In simple words call, operator function associate with delete operator not direct delete.
Let’s see what happen when we execute the code
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bhavith@bhavith:~$ ./a.out
Ctor called
Ctor called
Dtor called
munmap_chunk(): invalid pointer
Aborted (core dumped)
Did you see that? the application crashed and this is undefined behvaiour many of times
Let’s use gdb to debug it
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bhavith@bhavith:$ gdb a.out
GNU gdb (Ubuntu 12.0.90-0ubuntu1) 12.0.90
Copyright (C) 2022 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 "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
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For help, type "help".
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Reading symbols from a.out...
(No debugging symbols found in a.out)
(gdb) r
Starting program: /home/bhavith/Bhavith/Development/blogs/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Ctor called
Ctor called
Dtor called
munmap_chunk(): invalid pointer
Program received signal SIGABRT, Aborted.
__pthread_kill_implementation (no_tid=0, signo=6, threadid=140737348162496) at ./nptl/pthread_kill.c:44
44 ./nptl/pthread_kill.c: No such file or directory.
(gdb) bt
#0 __pthread_kill_implementation (no_tid=0, signo=6, threadid=140737348162496) at ./nptl/pthread_kill.c:44
#1 __pthread_kill_internal (signo=6, threadid=140737348162496) at ./nptl/pthread_kill.c:78
#2 __GI___pthread_kill (threadid=140737348162496, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3 0x00007ffff7b77476 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4 0x00007ffff7b5d7f3 in __GI_abort () at ./stdlib/abort.c:79
#5 0x00007ffff7bbe6f6 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff7d10b8c "%s\n") at ../sysdeps/posix/libc_fatal.c:155
#6 0x00007ffff7bd5d7c in malloc_printerr (str=str@entry=0x7ffff7d13230 "munmap_chunk(): invalid pointer") at ./malloc/malloc.c:5664
#7 0x00007ffff7bd605c in munmap_chunk (p=<optimized out>) at ./malloc/malloc.c:3060
#8 0x00007ffff7bda51a in __GI___libc_free (mem=<optimized out>) at ./malloc/malloc.c:3381
#9 0x000055555555527a in main ()
(gdb)
From the above debug message we can see program received SIGABRT i.e. abort signal from the kernel and if you back trace it, you can see that call trace is pointing from function main, and abort aroses when control reaches __GI___libc_free function.
Moral of the story:
Never delete the pointer pointing to any array of object like a regular pointer rather use delete[] p
Working source
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int main()
{
Test* p = new Test[2];
delete[] p; //Use this rather delete p
return 0;
}
Compile and rum
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bhavith@bhavith:~$ g++ main.cpp
bhavith@bhavith:~$ ./a.out
Ctor called
Ctor called
Dtor called
Dtor called