Consider the following program:

   int func();  main()  {    int n = 0;      int arr [2] = { func(), n}; /* ok in C++, not in C */  }

A C compiler will reject the declaration of arr because the initialization list doesn’t contain constant expressions. In contrast, a C++ compiler will blissfully compile it. You’re probably thinking that this isn’t an issue because you’re using a C++ compiler anyway. However, many C++ compilers rely on the source file’s extension to determine whether the code therein should be treated as C or C++.

A .c extension invokes the C compiler whereas a .cpp extension invokes the C++ compiler. Because the rules regarding initialization are somewhat different in both languages, the program above may or may not compile, depending on the file extension used. In C, the initializer list must contain constant expressions exclusively. In C++, you can use any valid expression as an initializer, including a function’s return value or a previously declared variable. Therefore, when porting legacy code, remember to check the file extensions as well.