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Thanks, it works
one more question
The routine 'myfunc' is designed to throw an exception of type 'myExcep'. However it is being called from the 'main' function assuming that an exception of type 'derivedExcep' will be caught. Correct the code so that the exception thrown by the 'myfunc' function is caught and handled by the catch block in the 'main' function. The routine 'my_handler' should be helpful to you.

#include <iostream>
#include <exception>
class myExcep {
public:
myExcep () {}
};
void myfunc() throw (myExcep) {
throw 0;
}
class derivedExcep : public myExcep {};

//changes allowed in below code only
void my_handler() throw (&amp;derivedExcep) {
std::cout<< "hit the handler" << std::endl;
throw derivedExcep();
}

int main() {
try {
myfunc();
}
catch (derivedExcep &amp; e) {
std::cout<<"caught exception gracefully"<< std::endl;
}
return 0;
}

you could add std::unexpected(my_handler) in main before calling myfunc.

'num' cannot be modified because it is being accessed through a const object

The keyword "mutable" is used to allow a particular data member of const object to be modified. It simply allows you to modify a variable in a 'const' method.
Most popularly used for Marking a boost::mutex member as mutable allowing const functions to lock it for thread-safety reasons