Default constructor

[al]

class Base
{
public:
//Base() {}
Base(int m) {}

};

class Derive : public Base
{
public:
Derive(int m) {}

};


The above code keeps giving compiler error: 'Base': no appropriate default
constructor available, until a default cstor, Base() is added into Base
class.

Does this mean that a default cstor has always to be added whenever a
non-default cstor present?

Or is this default cstor, Base(), only required during inheritance? How to
make Derive to call Base(int m) rather than Base()?

In general, when does a default cstor have to be provided?

Thanks!

[Adam Fineman]

Re: Default constructor

al wrote:[color=blue]
> class Base
> {
> public:
> //Base() {}
> Base(int m) {}
>
> };
>
> class Derive : public Base
> {
> public:
> Derive(int m) {}
>
> };
>
>
> The above code keeps giving compiler error: 'Base': no appropriate default
> constructor available, until a default cstor, Base() is added into Base
> class.[/color]

That's because your Derive contructor is attempting to construct a
default Base object, since you haven't told it how to initialize it's
Base component. Change your Derive contructor to this:

Derive(int m)
: Baee(m)
{
/* no code */
}

and it will compile without a default contructor for Base.
<snip>[color=blue]
> In general, when does a default cstor have to be provided?
>[/color]
Whenever an object must be initialized without any parameters.

- Adam

--
Reverse domain name to reply.

[Victor Bazarov]

Re: Default constructor

"al" <allin@168.ne t> wrote...[color=blue]
> class Base
> {
> public:
> //Base() {}
> Base(int m) {}
>
> };
>
> class Derive : public Base
> {
> public:
> Derive(int m) {}
>
> };
>
>
> The above code keeps giving compiler error: 'Base': no appropriate default
> constructor available, until a default cstor, Base() is added into Base
> class.[/color]

Correct. In the 'Derive's constructor you don't initialise the Base in any
way. The compiler has to do it itself. It uses default-initialisation.
Since you have a parameterised c-tor in Base, and no default c-tor, the
compiler cannot initialise the Base part of Derive.
[color=blue]
>
> Does this mean that a default cstor has always to be added whenever a
> non-default cstor present?[/color]

No.
[color=blue]
>
> Or is this default cstor, Base(), only required during inheritance? How[/color]
to[color=blue]
> make Derive to call Base(int m) rather than Base()?[/color]

class Derive : public Base
{
public:
Derive(int m) : Base(m) {}
};

BTW, what book are you using to learn C++ that doesn't tell you that?
[color=blue]
>
> In general, when does a default cstor have to be provided?[/color]

No.

Victor

[Dan Cernat]

Re: Default constructor

In addition:

if you provide no constructor, the compiler will generate a 'default' (no
parameters) constructor. As soon as you declare a constructor (with or
without parameters), the compiler will no longer generate the default for
you.

Dan