Why does `int ;` compile fine in C, but not in C++?

Question

Consider the following program (see live demo here).

#include 
int main(void)
{
      int ;  // Missing variable name
      puts(\"Surprise\")         


        

Answer 1

Your code is illegal (i.e. erroneous, ill-formed, constraint-violating) in both C and C++. The reason you get a "warning" in one language and "error" in another is just a quirk of your compiler and your compiler setup. After all, neither language really formally differentiates between "warnings" and "errors". GCC under its default settings just happens to be more permissive in C mode (mostly for historical reasons).

Use -pedantic-errors in GCC, and you will get an "error" in C code as well. (Note that -pedantic-errors does not simply blindly turn all "warnings" into "errors". It attempts to report only actual constraint violations as "errors".)