1. Headers should not include using declaration

Code inside headers ordinarily should not include using declarations. The reason is that the contents of a header are copied into the including program's text. If a header has a using declaration, then every program that includes that header gets the same using declaration. As a result, a program that does not intend to use the specified library name might encounter unexpected name conflicts.

2. Deal with characters in string( with functions in <cctype> header)

　　isalnum(c): true if c is a letter or digit

　　isalpha(c): true if c is a letter

　　isdigit(c): true if c is a digit

　　islower(c): true if c is a lowercase letter

　　isspace(c): true if c is a whitesapce

　　isupper(c): true if c is an uppercase letter

　　tolower(c): return lowercase letter

　　toupper(c): return uppercase letter

3. Different between pointer and iterator

Pointer is a built-in type whereas iterator is a class. Pointer is avaiable for all type of objects whereas iterator is only available for containers.

4. Vector and array

Similarity: they all store a collection of variables of a single type and we can access their element by position.

Difference: elements in array is stored in continuous space whereas elements in vector can be stored in non-continuous space. Array is fixed size and we can't add or remove element from a array. On the contrary, the size of a vector can be changed by adding or removing elements from it.

4. There is no arrays of references