Re: const keyword position

[Tamas Demjen <demjen@eik.bme.hu>]

Hi Tay,

The 'const' keyword is a type modifier that's good for a lot of things.
Its behavior depends on the context in which it appears. Here are some
examples:
1. Declaring constants
    const NumberOfItems = 10;
    In this case, NumberOfItems is NOT a variable. The compiler substitutes
    10 for NumberOfItems. Constants in C++ are similar to the #define
    definitions, but while #define's are processed in the first phase by the
    preprocessor, const's are processed in the second phase by the compiler.

2. Declaring a constant variable
    const int x = 10;
    Variable x cannot be written, it's read only. In most cases, const
    variables are used in a function parameter, for instance:
    int foo(const char* s);
    The foo function can't write the input string s, it can only read it.
    Note that the const refers not to the pointer itself. Variable s can
    still be incremented, decremented, or whatever, but *s cannot be an
    lvalue. So the const makes *s read-only. If you wanted to make the
    pointer read-only, you should have written char* const s. In this case,
    you could modify *s, but you wouldn't be able to modify s. To make both
    s and *s read-only, just write const char* const s.

3. Returning constant objects
    const char* foo();
    The foo function returns a constant variable. If you write *foo()='A',
    then you'll get an error message "Cannot modify a const object."

4. Constant member functions
    Consider the following class:
    class TClass
    {
    private:
       int x;
    public:
       TClass() { }
       void SetX(int value) { x = value; }
       int GetX() const { return x; }
    };
    SetX member function sets the private member variable x, while GetX
    returns x. GetX is a constant member function, because it doesn't modify
    any object in the class. Each function that doesn't modify any object in
    the class should be declared const. A const member function cannot write
    any member functions in the class. Only the const member functions can
    be called for a const object. Example:

    const TClass MyObject;
    MyObject.GetX();   // Ok
    MyObject.SetX(10); // error!

    Only member functions can be declared const. There is no such thing as
    const funtion.

I suggest you read The C++ Programming Language by Bjarne Stroustrup.

Tom