Is C++ backward compatible with ANSI/ISO-C?

Almost. C++ is as close as possible to compatible with C, but no closer. In practice, the major difference is that C++ requires prototypes, and that f() declares a function that takes no parameters (in C, f() is the same as f(...)). There are some very subtle differences as well, like sizeof('x') is equal to sizeof(char) in C++ but is equal to sizeof(int) in C. Also, C++ puts structure "tags" in the same namespace as other names, whereas C requires an explicit struct (e.g., the typedef struct Fred Fred; technique still works, but is redundant in C++).  more