throw <const string>

throw <const string>

[thomas joseph]

Hi All,
 I am trying a sample with c++ exception handling.
 Could you tell me the point in the following code.

#include <iostream>
using namespace std;
int main()
{
    char *buf = "Memory allocation failed!";

    try
    {
           throw "Memory allocation failure!";
          //  throw buf;
    }
    catch( char *str)
    {
        cout << "Exception raised: " << str << '\n';
    }
     return 0;
}


 If I compile this code and execute it gives me
segmentation fault. But If I comment the throw "Memory
..." and uncomment throw buf it works fine.


 What I can n't understand is both buf and Memory
Allocation are pointers. (point to read only data.).

Why is it failing in the first case and passing in
second case ?

(If I compile with -fwritable-strins both the
scenarios are fine.).

Thanks in advance,


--thomas













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Re: throw <const string>

[Miguel Ramírez]

Hi,

>
> Hi All,
>  I am trying a sample with c++ exception handling.
>  Could you tell me the point in the following code.
>
> #include <iostream>
> using namespace std;
> int main()
> {
>     char *buf = "Memory allocation failed!";
>
>     try
>     {
>            throw "Memory allocation failure!";
>           //  throw buf;
>     }
>     catch( char *str)
>     {
>         cout << "Exception raised: " << str << '\n';
>     }
>      return 0;
> }
>

C++ exceptions are objects:

#include <exception>

class MyCustomException : public std::exception
{
public:

const char* what()
{
    return " Memory Allocation failure";
}

};

so you can now

 #include <iostream>
#include "MyCustomException.hxx"
 using namespace std;
 int main()
 {
     try
     {
            throw "Memory allocation failure!";
           //  throw buf;
     }
     catch( std::exception& e )
     {
         cout << "Exception raised: " << e.what( ) << '\n';
     }
      return 0;
 }

Note that there are already many exceptions classes available in the
standard library,
so this example is kind of "rash". Well, it may be quite a pain in the back
to take account
of the standard exception class hierarchy, but it may pay out in the long
term.

Miguel Ramírez.

PS: Note that new already launches an exception when memory allocation fails
unless
you use the std::no_throw variant.

Re: throw <const string>

[Miguel Ramírez]

Sorry,

>             throw "Memory allocation failure!";

should read
            throw MyCustomException();

Too much cut & paste :(

RE: throw <const string>

[Moore, Mathew L]

>     char* buf = "Memory allocation failure!";
>     try
>     {
>            throw "Memory allocation failure!";
>           //  throw buf;
>     }
>     catch( char *str)
>     {
>         cout << "Exception raised: " << str << '\n';
>     }



A string literal in a throw statement is not converted to a char*.  For the
|throw "Memory allocation failure";|, you have to |catch(const char*)| for
this to work properly.  On the other hand, your |buf| variable is a char*,
because it is declared that way.  Hence, the |catch(char*)| works just fine
in the |throw buf;| case.

> 
>  If I compile this code and execute it gives me
> segmentation fault.


I guess I am also a little surprised it gives a seg fault.  


> 
> 
>  What I can n't understand is both buf and Memory
> Allocation are pointers. (point to read only data.).


|buf| is not read only.  You have to declare it that way.  

	const char* buf = "Memory allocation failure!";

You may want to consider using the standard library exceptions.

--Matt