'not declared in this scope' when using strlen()

You forgot to include <cstring> or <string.h>.

cstring will give you strlen in the std namespace, while string.h will keep it in the global namespace.

You need to include cstring header for strlen:

 #include <cstring>

you could alternatively include string.h and that would put strlen in the global namespace as opposed to std namespace. I think it is better practice to use cstring and to drop using using namespace std.