Recursion on main

[Oriol Serrabassa]

First of all, during this post keep in mind that when I am referring to a program or a code it means that the program is only able to do 1 thing or function or whatever you want to call it, thanks.

Is it possible to program any recursive function on main?
If so, I would like someone to post the solution to the following code.

#include <iostream>
using namespace std;

int operation () {
	char c;
	cin >> c;
	if (c == '+') return operation() + operation();
	if (c == '-') return operation() - operation();
	if (c == '*') return operation() * operation();
	else return c - '0';
}

int main () {
	cout << operation() << endl;
}

Is it recommended to use recursion on main or is it better to create a function for it?
The following 2 examples illustrate this case:

Example using recursion on a function.
- Less efficient
- More maneuverability and adaptability

#include <iostream>
using namespace std;

void turn () {
	string word;
	if (cin >> word and word != "end") {
		turn ();
		cout << word << endl;
	}
}

int main () {
	turn ();
}

Example using recursion on main:
- More efficient
- Less maneuverability and adaptability

#include <iostream>
using namespace std;

int main () {
	string word;
	if (cin >> word and word != "end") {
		main ();
		cout << word << endl;
	}
}

[jonnin]

it is not allowed in standard C++ but some compilers may support it.

Therefore the answer is to call a function.

[jlb]

Is it recommended to use recursion on main

No, calling main() in a C++ program is specifically not allowed by the C++ standard. Also, IMO, you should prefer iteration over recursion in most cases.

but some compilers may support it.

Then you should replace that broken compiler.

[jonnin]

Cygwin's g++ allowed it, lol. I thought it might so I tried it. Its always been a little off.

[JLBorges]

> g++ allowed it, lol. I thought it might so I tried it. Its always been a little off.

The GNU compiler is always a little off (everywhere) unless we explicitly demand conformance to the C++ standard.

-std=c++14 treat this this program as a C++ program; it is not a gnu++ program
-pedantic-errors Unfortunately, it wont believe you unless you also say that you were not joking when you said C++

http://coliru.stacked-crooked.com/a/812c6a2910884b14

[Oriol Serrabassa]

Thanks for the answers.
Why doesn't C++ allow it? Main is a function itself

[TheIdeasMan]

It is one of the ways of getting a stack overflow. A typical program puts it's data on the stack - there is limited room there. Recursively calling main, keeps adding a new frame to the stack, eventually causing the overflow.

[jlb]

And don't forget that main() is a special function that can call other "invisible" functions that preform startup duties, like setting up the standard input output streams and perhaps may even re-setup non-local variables, etc..

1
3.6 Start and termination [basic.start]
3.6.1 Main function [basic.start.main]
A program shall contain a global function called main, which is the designated start of the program. It is implementation-defined whether a program in a freestanding environment is required to define a main function. [ Note: In a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. — end note ]

Even in C, which doesn't specifically disallow recursively calling main(), the practice is discouraged.