I tested the following code with three different compilers and saw some peculiar results. First of all, I am not sure why the program compiles (no errors with isalpha) indicating that cctype/ctype.h is being automatically included allowing the compilation of the program.

As far as I know and according to cplusplus.com, neither cctype namespace nor ctype.h header file is amongst the automatically included namespace/header file for iostream. Also, iostream does not inherit from cctype/ctype.h.

#include <iostream>
using namespace std;

int main()
{
	cerr<<isalpha('a');
}

On my local machine with MinGW (32 bit), it prints 2.

On Codeforces compiler, it prints nothing.

On ideone, it prints 1024.

All the above compilations were done for C++14.

It would be great if someone could share the reason for this behaviour.