Question

I saw many examples but I am not able to understand, how to use try catch with a simple constructor, I wrote a sample program:

class A
 {
   public:
    try {
       A()
        { cout << "in costr\n"; throw 10;}
    }//try closed
   catch (int a)
{ cout << "caught 1 \n"; }

 };

main()
 {
   A *ptr = new A;
   }

1. This program gives a compilation error
2. If exception is caught, what happens to object ??

Answer 1

The try/catch code is supposed to be together, you can’t have one without the other. Something like this is what you’re after:

A *ptr;
try {
    ptr = new A();
} catch (int a) {
    cout << "caught 1\n";
}

See the following program for a complete working example:

#include <iostream>

class A {
    private:
        int a;
    public:
        A() { a = 7; throw 42; }
        int getA() { return a; }
};

int main (void) {
    A *ptr;
    try {
        ptr = new A();
    } catch (int b) {
        std::cout << "Exception: " << b << '\n';
        return -1;
    }
    std::cout << "Value: " << ptr->getA() << '\n';
    return 0;
}

With the throw 42 in there, you see:

Exception: 42

meaning that main has caught the exception coming from the constructor. Without the throw, you see:

Value: 7

because everything has worked.

---

The main problems with your code seem to be:

• You have a try statement where it shouldn’t be. Try/catch blocks should generally be within a function or method, you have it immediately after the public keyword.

• If you’re throwing an exception from the constructor, you don’t catch it in the constructor. Instead you catch it in the code that called the constructor (main in this case).

• As previously mentioned, try and catch go together, they’re not standalone entities.

---

If you are trying to throw and catch within the constructor, you’ll still need to put it within the constructor itself, something like:

#include <iostream>

class A {
    private:
        int a;
    public:
        A() {
            try {
                a = 7;
                throw 42;
            } catch (int b) {
                std::cout << "Exception A: " << b << '\n';
                throw;
            }
        }
        int getA() {return a;}
};

int main(void) {
    A *ptr;
    try {
        ptr = new A();
    } catch (int b) {
        std::cout << "Exception B: " << b << '\n';
        return -1;
    }
    std::cout << "Value: " << ptr->getA() << '\n';
    return 0;
}

which gives you:

Exception A: 42
Exception B: 42

Note specifically how the try/catch block is both complete and within the constructor function.