Why do we need to use `int main` and not `void main` in C++? [duplicate]

The short answer, is because the C++ standard requires main() to return int.

As you probably know, the return value from the main() function is used by the runtime library as the exit code for the process. Both Unix and Win32 support the concept of a (small) integer returned from a process after it has finished. Returning a value from main() provides one way for the programmer to specify this value.

Most Operating Systems report back to the user, or the calling process, if an application was successful or not. This is especially useful in scripting, where the script can conditionally branch (if-then) on the results of a program. Something along the lines of:

// pseudo-code
screenscrape  http://mydatasource.com > results.txt
if errorlevel == 0 then
   processfile results.txt
else
   echo Screen Scraping Failed!
end if

This result status is done via the return value of main.

While some compilers allow for void main, for the sake of consistency and simplicity, the ANSI standard requires one single prototype of main:

int main(int argc, char *argv[]);

Because in C, arguments are cleaned up by the caller, the author of main can neglect to declare or process the arguments argc & argv. However, if the setup-routines that call main expect an int return value, and instead don't find one, behavior can undefined.

Short answer:

• The return value of main is useful for scripting.
• The setup and cleanup routines that invoke main need a consistent interface to use.

Main reason for changing

void main() { }

to

int main() { }

in later releases was to notify error occurred in program during execution to operating system on which it running

return 0;

identify program successfully executed if any number rather then 0 returned that means some error occurred who's error code is which returned by main. if you are running on codeblock IDE see in build log if main return 0 it normally display

Process terminated with status 0

else it display status code in red which means an error occurred

From Wikipedia:

The value returned from the main function becomes the exit status of the process, though the C standard only ascribes specific meaning to two values: EXIT_SUCCESS (traditionally zero) and EXIT_FAILURE. The meaning of other possible return values is implementation-defined.