virtual function

[hugheslin@hotmail.com]

Hi,

Please consider the following classes:

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
class Shape
{
......
......
public:
virtual void draw(CDC & aDC, COLORREF color, COLORREF fcolor, int
width, BOOL Filled = false) = 0;
};

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
class Line : public Shape
{
......
......
public:
void draw(CDC &dc, COLORREF color, COLORREF fcolor, int width,
BOOL Filled = false);
}
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
class ellipse : public Shape
{
......
......
public:
void draw(CDC & dc, COLORREF color, COLORREF fcolor, int width,
BOOL Filled = false);
};
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

My question is:

If I declared the member function "draw( )" inside the classes and
implemented the definitions of "draw( )" outside the classes, then
there will be an error messsage when compiling :

"redefiniti on of default parameter : parameter 5"

However, if I implemented all the definitions of "draw( )" inside the
classes, then the code can pass the compilation.

Would anybody please let me know what the issue is? Thakns a lot.

[Gavin Deane]

Re: virtual function


hugheslin@hotma il.com wrote:[color=blue]
> Hi,
>
> Please consider the following classes:
>
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> class Shape
> {
> .....
> .....
> public:
> virtual void draw(CDC & aDC, COLORREF color, COLORREF fcolor, int
> width, BOOL Filled = false) = 0;
> };
>
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
>
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> class Line : public Shape
> {
> .....
> .....
> public:
> void draw(CDC &dc, COLORREF color, COLORREF fcolor, int width,
> BOOL Filled = false);
> }
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
>
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> class ellipse : public Shape
> {
> .....
> .....
> public:
> void draw(CDC & dc, COLORREF color, COLORREF fcolor, int width,
> BOOL Filled = false);
> };
> /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
>
> My question is:
>
> If I declared the member function "draw( )" inside the classes and
> implemented the definitions of "draw( )" outside the classes, then
> there will be an error messsage when compiling :
>
> "redefiniti on of default parameter : parameter 5"
>
> However, if I implemented all the definitions of "draw( )" inside the
> classes, then the code can pass the compilation.[/color]

A much simpler example of the same problem is the following

// Function declaration
void foo(int i = 42);

int main()
{
return 0;
}

// Function definition
void foo(int i = 42) {}

You are not allowed to define the default parameter in two places. The
solution is either

// Function declaration and definition
void foo(int i = 42) {}

int main()
{
return 0;
}

which is the equivalent of you defining your draw functions inside your
classes, or

// Function declaration
void foo(int i = 42) {}

int main()
{
return 0;
}

// Function definition
// Note default parameter not redefined
void foo(int i) {}

Gavin Deane

[Neelesh Bodas]

Re: virtual function


hugheslin@hotma il.com wrote:[color=blue]
> Hi,
>
> Please consider the following classes:
>[/color]
[color=blue]
> class Shape[/color]
{> public:[color=blue]
> virtual void draw(CDC & aDC, COLORREF color, COLORREF fcolor, int
> width, BOOL Filled = false) = 0;
> };
> class Line : public Shape
> {
> public:
> void draw(CDC &dc, COLORREF color, COLORREF fcolor, int width,
> BOOL Filled = false);
> }[/color]
[color=blue]
> My question is:
>
> If I declared the member function "draw( )" inside the classes and
> implemented the definitions of "draw( )" outside the classes, then
> there will be an error messsage when compiling :
>
> "redefiniti on of default parameter : parameter 5"[/color]

Actually, your question has nothing to with "virtual functions". C++
doesnot allow redefinition of default arguments of functions. So once
they are defined in the function prototype, you should not specify them
again in the function definition.
<quote>
8.3.6p4 : A default argument shall not be redefined by a later
declaration (not even to the same value).
</quote>

[Fiesta]

Re: virtual function

Hi Gavin,

Thanks a lot for your suggestion and it did help. Thanks again.


Gavin Deane wrote:[color=blue][color=green]
> > Hi,
> >
> > Please consider the following classes:
> >
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> > class Shape
> > {
> > .....
> > .....
> > public:
> > virtual void draw(CDC & aDC, COLORREF color, COLORREF fcolor, int
> > width, BOOL Filled = false) = 0;
> > };
> >
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> >
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> > class Line : public Shape
> > {
> > .....
> > .....
> > public:
> > void draw(CDC &dc, COLORREF color, COLORREF fcolor, int width,
> > BOOL Filled = false);
> > }
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> >
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> > class ellipse : public Shape
> > {
> > .....
> > .....
> > public:
> > void draw(CDC & dc, COLORREF color, COLORREF fcolor, int width,
> > BOOL Filled = false);
> > };
> > /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
> >
> > My question is:
> >
> > If I declared the member function "draw( )" inside the classes and
> > implemented the definitions of "draw( )" outside the classes, then
> > there will be an error messsage when compiling :
> >
> > "redefiniti on of default parameter : parameter 5"
> >
> > However, if I implemented all the definitions of "draw( )" inside the
> > classes, then the code can pass the compilation.[/color]
>
> A much simpler example of the same problem is the following
>
> // Function declaration
> void foo(int i = 42);
>
> int main()
> {
> return 0;
> }
>
> // Function definition
> void foo(int i = 42) {}
>
> You are not allowed to define the default parameter in two places. The
> solution is either
>
> // Function declaration and definition
> void foo(int i = 42) {}
>
> int main()
> {
> return 0;
> }
>
> which is the equivalent of you defining your draw functions inside your
> classes, or
>
> // Function declaration
> void foo(int i = 42) {}
>
> int main()
> {
> return 0;
> }
>
> // Function definition
> // Note default parameter not redefined
> void foo(int i) {}
>
> Gavin Deane[/color]