A number of the answers here cover syntax differences that would cause C++ compilers to fail on C89 (or C99) source code. However, there are some subtle language differences that are legal in both languages but that would produce different behavior. The sizeof (char) difference that Naveen mentioned is one example, but Write a program that will print "C" if compiled as an (ANSI) C program, and "C++" if compiled as a C++ program lists some others.
