How to create a class that casting to int explicitly only?

[Tao Wang]

Hi,

I have a class Data as following

class Data{
....
};

I know if I want to cast it to int implicitly I should write like
following:

class Data{
....
public:
operator int() {...};
};

So, I can use the class like,

Data d(1234);
int a = d;

However, what I really want is that user of class Data have explicitly
cast it to int.

that is, user should write the code like:

int a = (int) d;

other than:

int a = d;

How can I make the cast to int explicitly only?

Thanks.

Tao Wang

[Nick Keighley]

On 30 Jul, 10:18, Tao Wang <Dancef...@gmai l.comwrote:

Hi,

I have a class Data as following

class Data{
...

};

I know if I want to cast it to int implicitly I should write like
following:

class Data{
...
public:
* *operator int() {...};

};

So, I can use the class like,

Data d(1234);
int a = d;

However, what I really want is that user of class Data have explicitly
cast it to int.

that is, user should write the code like:

int a = (int) d;

other than:

int a = d;

How can I make the cast to int explicitly only?

does this do what you want?

class Data
{
public:
explicit Data(int);
};

--
Nick Keighley

[blargg]

In article
<e2************ *************** *******@a2g2000 prm.googlegroup s.com>,
Tao Wang <Da*******@gmai l.comwrote:

...
class Data{
...
public:
operator int() {...};
};

So, I can use the class like,

Data d(1234);
int a = d;

However, what I really want is that user of class Data have explicitly
cast it to int.

that is, user should write the code like:

int a = (int) d;

other than:

int a = d;

How can I make the cast to int explicitly only?

class Data{
....
public:
int to_int() {...};
};

Data d(1234);
int a = (int) d; // error
int b = d.to_int(); // OK

[ManicQin]

On Jul 30, 11:28*am, Nick Keighley <nick_keighley_ nos...@hotmail. com>
wrote:

>
does this do what you want?

class Data
{
* * public:
* * * * explicit Data(int);

};

1 )I'm not sure You can add explicit to a casting operator.
2) If it does work it will still wont do what he wants...

I think blargg is right...

[Lionel B]

On Wed, 30 Jul 2008 03:07:01 -0700, ManicQin wrote:

On Jul 30, 11:28Â*am, Nick Keighley <nick_keighley_ nos...@hotmail. com>
wrote:

>does this do what you want?

class Data
{
Â* Â* public:
Â* Â* Â* Â* explicit Data(int);

};

1 )I'm not sure You can add explicit to a casting operator.

Well, that is a constructor, not a casting operator.

2) If it does work it will still wont do what he wants...

Right. The `explicit' as above will prevent assignment of Data objects by
ints (more or less the opposite of what the OP wants) as in:

Data d;
...
d = 3; // error: no match for ‘operator=’ in ‘d = 3’

I think blargg is right...

Maybe not precisely what the OP requested, but seems sensible enough.

--
Lionel B

[Tao Wang]

On Jul 30, 7:28 pm, Nick Keighley <nick_keighley_ nos...@hotmail. com>
wrote:

On 30 Jul, 10:18, Tao Wang <Dancef...@gmai l.comwrote:

Hi,

I have a class Data as following

class Data{
...

};

I know if I want to cast it to int implicitly I should write like
following:

class Data{
...
public:
operator int() {...};

};

So, I can use the class like,

Data d(1234);
int a = d;

However, what I really want is that user of class Data have explicitly
cast it to int.

that is, user should write the code like:

int a = (int) d;

other than:

int a = d;

How can I make the cast to int explicitly only?

does this do what you want?

class Data
{
public:
explicit Data(int);

};

--
Nick Keighley

No, it is in wrong direction.

What I expected result is

Data d(1234);

int a = d; // error
int a = (int) d; // OK

Your solution is provide explicitly casting FROM int, rather than to
int.

How this be done?

[Tao Wang]

On Jul 30, 7:46 pm, blargg <blargg....@gis hpuppy.comwrote :

In article
<e23c84a3-9bfa-4c5d-87f2-eed0cb157...@a2 g2000prm.google groups.com>,
Tao Wang <Dancef...@gmai l.comwrote:

...
class Data{
...
public:
operator int() {...};
};

So, I can use the class like,

Data d(1234);
int a = d;

However, what I really want is that user of class Data have explicitly
cast it to int.

that is, user should write the code like:

int a = (int) d;

other than:

int a = d;

How can I make the cast to int explicitly only?

class Data{
...
public:
int to_int() {...};

};

Data d(1234);
int a = (int) d; // error
int b = d.to_int(); // OK

This is similar to what I expected, however, I don't want user to
remember the interface to_int(). What I expect is that the code:

Data d(1234);
int a = (int) d; // OK

should be correct, that is, Data can convert to int, however, I don't
want it convert implicitly.

[Victor Bazarov]

Tao Wang wrote:

On Jul 30, 7:46 pm, blargg <blargg....@gis hpuppy.comwrote :

>[..]
Data d(1234);
int a = (int) d; // error
int b = d.to_int(); // OK

This is similar to what I expected, however, I don't want user to
remember the interface to_int(). What I expect is that the code:

Data d(1234);
int a = (int) d; // OK

should be correct, that is, Data can convert to int, however, I don't
want it convert implicitly.

There is *no way* to do what you want. And try not to repeat yourself.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Noah Roberts]

Tao Wang wrote:

On Jul 30, 7:46 pm, blargg <blargg....@gis hpuppy.comwrote :

>In article
<e23c84a3-9bfa-4c5d-87f2-eed0cb157...@a2 g2000prm.google groups.com>,
Tao Wang <Dancef...@gmai l.comwrote:

>>...
class Data{
...
public:
operator int() {...};
};
So, I can use the class like,
Data d(1234);
int a = d;
However, what I really want is that user of class Data have explicitly
cast it to int.
that is, user should write the code like:
int a = (int) d;
other than:
int a = d;
How can I make the cast to int explicitly only?

class Data{
...
public:
int to_int() {...};

};

Data d(1234);
int a = (int) d; // error
int b = d.to_int(); // OK

This is similar to what I expected, however, I don't want user to
remember the interface to_int(). What I expect is that the code:

Data d(1234);
int a = (int) d; // OK

should be correct, that is, Data can convert to int, however, I don't
want it convert implicitly.

Well, if you stop using C style casts, which you should do anyway, then
you can create for yourself a new kind of cast. Several classes do this
in the boost library. Then they just have to use your new cast. If you
use this cast for other objects though they'll find it easy to remember.

In other words, make an external function but call it something that
ends in "_cast".

In fact, you could even piggy back boost::lexical_ cast if you really
wanted to. Just make an operator >(Data & in, int & out).

Really though, this problem of yours could indicate a bad design
decision. It shouldn't be unreasonable to expect the user to know the
function call they need to make to get the int out of the object.

[Noah Roberts]

Noah Roberts wrote:

In fact, you could even piggy back boost::lexical_ cast if you really
wanted to. Just make an operator >(Data & in, int & out).

Actually, I was being a dumbass. This won't work. You probably don't
want anything to do with lexical cast if speed is even remotely a concern.