The definition

void main()
{ /* ... */ }
is not and never has been C++, nor has it even been C. See the ISO C++ standard 3.6.1[2] or the ISO C standard 5.1.2.2.1. A conforming implementation accepts

int main()
{ /* ... */ }
and

• int main(int argc, char* argv[])
  { /* ... */ }
  A conforming implementation may provide more versions of main(), but they must all have return type int. The int returned by main() is a way for a program to return a value to “the system” that invokes it. On systems that doesn’t provide such a facility the return value is ignored, but that doesn’t make “void main()” legal C++ or legal C. Even if your compiler accepts “void main()” avoid it, or risk being considered ignorant by C and C++ programmers.
• In C++, main() need not contain an explicit return statement. In that case, the value returned is 0, meaning successful execution.

For example:

#include <iostream>
int main()
{
std::cout
<< "This program returns the integer value 0n";
}

• Note also that neither ISO C++ nor C99 allows you to leave the type out of a declaration. That is, in contrast to C89 and ARM C++ ,”int” is not assumed where a type is missing in a declaration.

Consequently:

#include <iostream>

main()
{ /* ... */}