C and C++ Question

Why must you declare functions twice?

Actually, in C, you only have to declare a function if its return value or one of its arguments is not a word-sized integer. That's a feature inherited from the B programming language (which had the word-sized integer as its only type), I guess for compatibility. That's also why if you don't specify the type of a variable it defaults to int.

>>17
$ cat > noproto.c
main() { return !printf("%d\n", times2(16)); }
times2(a) { return mul(a, 2); }
mul(a, b) { return a * b; }
$ gcc -w -o noproto noproto.c && ./noproto
32
$