Base class catch not catching exception even if it is appearing before derived class catch

Question

Please see the expected flow of below code

In this case base class exception is catching as this expected behavior due to its polymorphic nature.

#include <stdio.h>
#include<iostream.h>
using namespace std;
void main() {

    try {
        //throw CustomException();
        throw bad_alloc();
    }
    ///due to poly morphism base class reference will
    catch(exception &ex) {
        cout<<"Base :"<<ex.what()<<endl;
    }

    catch(bad_alloc &ex) {
        cout<<"Derieved :"<<ex.what()<<endl;
    }
}

output

 bad_alloc

But if I am making a custom exception like shown in the below code, the derived class is catching the exception, even if the base class catch is appearing first in the catch blocks:

class CustomException : exception {

public :
     CustomException() { }

     const char * what() const throw() {
         // throw new exception();
         return "Derived Class Exception !";
     }
};

void main() {
    try {
        throw CustomException();
        //throw bad_alloc();
    }
    ///due to poly morphism base class reffrence will
    catch(exception &ex) {
        cout<<"Base :"<<ex.what()<<endl;
    }

    catch(CustomException &ex) {
        cout<<"Derived :"<<ex.what()<<endl;
    }
} 

expected output :

Base : Derived Class Exception !

Actual output:

Derived: Derived Class Exception !

Answer 1

The default inheritance access specifier for classes declared using the class keyword is private. This means that CustomException inherits from exception privately. A derived class that uses private inheritance can't be bound to a reference to its parent class.

If you inherit publicly it will work fine:

class CustomException : public exception // <-- add public keyword here
{
public :
     CustomException()
     {
     }

     const char * what()
     {
         return "Derived Class Exception !";
     }
};

int main()
{

    try
    {
        throw CustomException();
    }
    catch(exception &ex)
    {
        cout<<"Base :"<<ex.what()<<endl;
    }

    catch(CustomException &ex)
    {
        cout<<"Derived :"<<ex.what()<<endl;
    }
} 

Live Demo