Question

I have read several articles here and else where that it is OK to throw exception from constructor. However, I have noticed that it doesn’t call destructor of base class or its data members if an exception is thrown from the constructor. Consider the following example:

#include <iostream>
using namespace std;
struct C
{
    C() { cout << __FUNCTION__ << endl;  }
    ~C() { cout << __FUNCTION__ << endl; }
};

struct E: public C
{
    C c;
    E() { cout << __FUNCTION__ << endl; throw 4; }
    ~E() { cout << __FUNCTION__ << endl; }
};

int main()
{
    E e;
}


$ g++ test.cpp; ./a.exe
C
C
E
terminate called after throwing an instance of 'int'
Aborted (core dumped)

In this case, E’s constructor throws an exception but C’s destructor as a data member or as a base class is not called. Now if C’s destructor performs some cleanup operation like closing files/sockets and deleting heap allocations, this can cause problems.

So my question is why and when is it OK to throw exceptions from constructors.

Answer 1

If you catch the error, the destructor will be run. When an uncaught exception is thrown in C++, the runtime calls std::terminate. By default, std::terminate calls std::abort which specifically does not call destructors on the way out.

With this version:

#include <iostream>
using namespace std;
struct C
{
    C() { cout << __FUNCTION__ << endl;  }
    ~C() { cout << __FUNCTION__ << endl; }
};

struct E: public C
{
    C c;
    E() { cout << __FUNCTION__ << endl; throw 4; }
    ~E() { cout << __FUNCTION__ << endl; }
};

int main()
{
    try {
        E e;
    } catch(...) {
    }

    return 0;
}

I get output:

C
C
E
~C
~C