Can I use if (pointer) instead of if (pointer != NULL)?

Yes, you could.

• A null pointer is converted to false implicitly
• a non-null pointer is converted to true.

This is part of the C++ standard conversion, which falls in Boolean conversion clause:

§ 4.12 Boolean conversions

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

You can; the null pointer is implicitly converted into boolean false while non-null pointers are converted into true. From the C++11 standard, section on Boolean Conversions:

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true . A prvalue of type std::nullptr_t can be converted to a prvalue of type bool ; the resulting value is false .

Yes, you can. In fact, I prefer to use if(pointer) because it's simpler to read and write once you get used to it.

Also note that C++11 introduced nullptr which is preferred over NULL.