2008 January 18th Friday (一月　十八日　金曜日)

typeid

  The typeid operator lets you determine if two objects are the same type. Somewhat like sizeof, it accepts two kinds of arguments:

  * The name of a class

  * An expression that evaluates to an object

  The typeid operator returns a reference to a type_info object, where type_info is a class defined in the typeinfo header file
(formerly typeinfo.h). The type_info class overloads the == and != operators so that you can use these operators to compare types.

A function uses passed data without modifying it:

  If the data object is small, such as a built-in data type or a small structure, pass it by value.

  If the data object is an array, use a pointer because that's your only choice. Make the pointer a pointer to const.

  If the data object is a good-sized structure, use a const pointer or a const reference to increase program efficiency. You save
  the time and space needed to copy a structure or a class design. Make the pointer or reference const.

  If the data object is a class object, use a const reference. The semantics of class design often require using a reference, which
  is the main reason why C++ added this feature. Thus, the standard way to pass class object arguments is by reference.

class string {
public:

  ...

private:
  char *data;

  mutable size_t datalength;
                                  //mutable；can use them
  mutable bool lengthisvalid;     //even if in a const function
};

size_t string::length() const
{
  if (!lengthisvalid) {
    datalength = strlen(data);    // ok
    lengthisvalid = true;         // ok
  }

  return datalength;
}

Nested Quasiquote Forms Evalation Rule

  Quasiquote forms may be nested.  Substitutions are made only for unquoted components appearing at the same nesting
level as the outer most back quote.