hi(){ }

g++ -x c++ -c test1.cpp
test1.cpp:1: ISO C++ forbids declaration of `hi' with no 
type

gcc -x c -c test1.cpp
Here the default return type is int.

<code>
int class;
</code>

$gcc a.c -c; echo $?
0

$g++ a.c -c; echo $?
a.c:1: error: expected identifier before ';' token
a.c:1: error: multiple types in one declaration
a.c:1: error: declaration does not declare anything
1

xyz()
{
printf("very good");
}

the above code will compile in 'c'
but in 'c++' it will give compilation error

xyz()
{
printf("very good");
}

this is correct and if u add getch();
in main then there is no chance of compilation in CPP for sure

ans 3 and 4 are incorrect in my opinion. 

xyz()
{
   printf("very good");
}
can be compiled by including the header file "Stdio.h"

void main()
{
   int class=28;
   printf("%d",class);
}


it works... 

[class] :p