double conversion

Xiangbuilder · 10-07-2003, 01:01 AM

Here is a program from static_cast sector, chapter 2 of <<thinking in c++>>,
why I can't compile it successfully?

Code:

#include<iostream>
using namespace std;

int main ()
{   
   double d=0.0;
   int x=d; 
   cout<<"x="<<x<<endl;
}
[root@localhost a_static]# g++ static_cast_i.cpp -o static_cast_i
static_cast_i.cpp: In function `int main()':
static_cast_i.cpp:7: warning: initialization to `int' from `double'
static_cast_i.cpp:7: warning: argument to `int' from `double'
[root@localhost a_static]#

thank you.

nephilim · 10-07-2003, 04:32 AM

try to change your int main() to

void main(String[] args)

and see what happens.

Xiangbuilder · 10-07-2003, 08:09 AM

Thank you.

Code:

[root@localhost a_static]# g++ static_cast_i.cpp -o static_cast_i
static_cast_i.cpp:4: parse error before `)' token
static_cast_i.cpp:5: `main' must return `int'
static_cast_i.cpp: In function `int main(...)':
static_cast_i.cpp:7: warning: initialization to `int' from `double'
static_cast_i.cpp:7: warning: argument to `int' from `double'
[root@localhost a_static]#

maybe the compiler cause the error. my guess.

nephilim · 10-07-2003, 08:25 AM

As I said before, you should change the "int" to "void". The compiler expects the method to return an integer, but you don't return anything.

dakensta · 10-07-2003, 08:29 AM

You have compiled sucessfully, those are compiler warnings because you are initialising an integer with a double, with the obvious potential for loss of information, without telling the compiler explicitly that you know what you are doing.

Which is what a cast does.

nephilim · 10-07-2003, 08:53 AM

Quote:

You have compiled sucessfully,...

Not true. It is true you can overlook the warnings, but not the errors.

As long as the int return type isn't changed, the code will never compile.

Xiangbuilder · 10-07-2003, 09:53 AM

hi, nephilim,
"you should change the "int" to "void"."
I had changed it before I compiled it again.

Code:

#include<iostream>
using namespace std;
void main (String[] args)
{   
   double d=0.0;
   int x=d; 
   cout<<"x="<<x<<endl;
}
[root@localhost a_static]# g++ static_cast_i.cpp -o static_cast_i
static_cast_i.cpp:4: `String' was not declared in this scope
static_cast_i.cpp:4: parse error before `]' token
static_cast_i.cpp:5: `main' must return `int'
static_cast_i.cpp: In function `int main(...)':
static_cast_i.cpp:7: warning: initialization to `int' from `double'
static_cast_i.cpp:7: warning: argument to `int' from `double'
[root@localhost a_static]#

*** ***

hi, dakensta,
After I compiled the program that I mentioned in my first poist,
although there are some warnning information, but a executable file, static_cast_j had been created.
here is the result when I run it.
[root@localhost a_static]# ./static_cast_j
x=0
[root@localhost a_static]#

*** ***

Thank you, nephilim and dakensta.

Xiangbuilder · 10-07-2003, 09:59 AM

hi, nephilim,
"The compiler expects the method to return an integer, but you don't return anything."
I don't think so.
if there is no return comman, the compiler, g++ can consider it as return 0;

besides, a main function must return a integer value instead of void (nothiing), it is new rule of c++.

thank you.

jinksys · 10-07-2003, 12:02 PM

The code you posted first,

Code:

#include<iostream>
using namespace std;

int main ()
{   
   double d=0.0;
   int x=d; 
   cout<<"x="<<x<<endl;
}

was correct. As stated, your compiler just warns you of
the type conversion.

If you want a program that compiles clean, with no errors, add a type cast like so:

Code:

#include<iostream>
using namespace std;

int main ()
{   
   double d=0.0;
   int x=(int)d; 
   cout<<"x="<<x<<endl;
}

Mohsen · 10-07-2003, 12:34 PM

Quote:

static_cast_i.cpp: In function `int main()':
static_cast_i.cpp:7: warning: initialization to `int' from `double'
static_cast_i.cpp:7: warning: argument to `int' from `double'

Yes, they are all warnings.
And about "main (String[])", AFAIK there is no String implemented for C++. You are complexing JAVA and C++

. But also in JAVA there is no `String' but `string'.

jinksys · 10-07-2003, 01:09 PM

{Edited}

nephilim · 10-07-2003, 01:14 PM

Mohsen,
You are correct, I was thinking in Java and not in C++. My most humble appologies

But in Java, it definitely is String and not string (String is an object and not a primitive). However, that's not the issue here.

jinksys,
I wasn't smoking anything. Maybe you should grab a dictionary and look up the word "polite".

jinksys · 10-07-2003, 02:09 PM

Yeah, that was pretty immature. Accept my apologies.

nephilim · 10-09-2003, 02:52 AM

Accepted, reading it all again I feel I overreacted.

No hard feelings then.

mr_segfault · 10-09-2003, 07:53 AM

Is it better (and more portable?), to use the type safe casts like:

Code:

#include<iostream>
using namespace std;

int main ()
{   
   double d=0.0;
   int x=static_cast<int>(d); 
   cout<<"x="<<x<<endl;
}

I have been led to believe that the new style type safe casts should be preferred to the old C style casts, since the type safe casts (static_cast<>, dynamic_cast<>) allow the compiler to determin if the coversion is safe to perform, where as the C style casts tell the compiler to "believe me, its ok"?

Cheers.