I can´t understand it!

[Jose Alberto]

Hi,

Is it possible a implicit cast from const char * to bool?


With this program:

class Foo
{
public:
pru (string param){ cout <<"string"<<end l;}
pru (bool param){ cout <<"bool";<<endl }
};

int main()
{
pru * p1=new pru("string");
string pp = "string";
pru * p2=new pru(pp);
pru * p3=new pru(true);
}

I get this results:

bool
string
bool

So in the first new, when I pass a const char type the compiler is making a implicit cast to bool. Anybody can give me an explanation?

TIA.
Jose.

[Peter van Merkerk]

Re: I can´t understand it!

Jose Alberto wrote:[color=blue]
> Hi,
>
> Is it possible a implicit cast from const char * to bool?
>
>
> With this program:
>
> class Foo
> {
> public:
> pru (string param){ cout <<"string"<<end l;}
> pru (bool param){ cout <<"bool";<<endl }
> };
>
> int main()
> {
> pru * p1=new pru("string");
> string pp = "string";
> pru * p2=new pru(pp);
> pru * p3=new pru(true);
> }
>
> I get this results:
>
> bool
> string
> bool
>
> So in the first new, when I pass a const char type the compiler is
> making a implicit cast to bool. Anybody can give me an explanation?[/color]

Since the is no constructor that takes a const char* as input argument, so
a conversion has to made when you try to construct a Foo object using
"string" as argument for the constructor. A const char* can be converted to
both a bool and a string. In this case the conversion to bool wins.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

[Rolf Magnus]

Re: I can?t understand it!

Jose Alberto wrote:
[color=blue]
> Hi,
>
> Is it possible a implicit cast from const char * to bool?[/color]

Yes.
[color=blue]
> With this program:
>
> class Foo
> {
> public:
> pru (string param){ cout <<"string"<<end l;}
> pru (bool param){ cout <<"bool";<<endl }
> };
>
> int main()
> {
> pru * p1=new pru("string");
> string pp = "string";
> pru * p2=new pru(pp);
> pru * p3=new pru(true);
> }
>
> I get this results:
>
> bool
> string
> bool
>
> So in the first new, when I pass a const char type the compiler is
> making a implicit cast to bool. Anybody can give me an explanation?[/color]

You can write something like:

const char* p = whatever;
if (p)
do_something();
else
do_something_el se();

But an if statement wants a bool, so the pointer p must be implicitly
convertible to bool. So p is converted to a bool with the value false
if it's a null pointer, or true otherwise.
So in your above example, both overloads are candidates for the call.
The bool overload is chosen because bool is a built-in type, while
string is a user-defined type, and conversions to built-in types are
always preferred.
You have to add another overload for const char*.