KwCheng's blog

const通常代表的就是宣告該物件不可以變動 , 

像 const a = 3 , 就是宣告a為常數3 , 不可以更動 .

而 function (const a , const b) , 就是說在這個function裡面 , 不可以變動到a與b的值.

另外 , 像

void A::foo() const 

{

cout << "I : " << _i << endl;

}

又稱為const member function , 其意義就是限制該function只能取得資料 , 不可以改變資料 (read-only) , 

另外 , 在這個const member function中 , 也不能呼叫任何不是 const 的member function.

Declaring a member function with the const keyword specifies that the function is a "read-only" function

 that does not modify the object for which it is called. A constant member function cannot modify any non-static data members 

or call any member functions that aren't constant.

// constant_member_function.cpp
class Date
{
public:
   Date( int mn, int dy, int yr );
   int getMonth() const;     // A read-only function
   void setMonth( int mn );   // A write function; can't be const
private:
   int month;
};

int Date::getMonth() const
{
   return month;        // Doesn't modify anything
}
void Date::setMonth( int mn )
{
   month = mn;          // Modifies data member
}
int main()
{
   Date MyDate( 7, 4, 1998 );
   const Date BirthDate( 1, 18, 1953 );
   MyDate.setMonth( 4 );    // Okay
   BirthDate.getMonth();    // Okay
   BirthDate.setMonth( 4 ); // C2662 Error
}

ref : http://msdn.microsoft.com/zh-tw/library/6ke686zh.aspx