Destructor called after throwing from a constructor

Question

I used to think that in C++, if a constructor throws an exception, the destructor of this "partially constructed" class is not called.

But it seems that it is not true anymore in C++11: I compiled the following code with g++ and it prints "X destructor" to the console. Why is this?

#include <exception> #include <iostream> #include <stdexcept> using namespace std;  class X { public:     X() : X(10)     {         throw runtime_error("Exception thrown in X::X()");         }     X(int a)     {         cout << "X::X(" << a << ")" << endl;     }     ~X()     {         cout << "X destructor" << endl;     } };  int main() {     try     {         X x;     }     catch(const exception& e)     {         cerr << "*** ERROR: " << e.what() << endl;     } } 

Output

Standard out: X::X(10)  X destructor Standard error:  *** ERROR: Exception thrown in X::X() 

Answer 1

Delegating constuctors are indeed a new feature that introduces a new destruction logic.

Let us revisit the lifetime of an object: An object's lifetime begins when some constructor has finished. (See 15.2/2. The standard calls this the "principal constructor".) In your case, this is the constructor X(int). The second, delegating constructor X() acts as just a plain member function now. Upon scope unwinding, the destructors of all fully-constructed objects are called, and this includes x.

The implications of this are actually quite profound: You can now put "complex" work loads into a constructor and take full advantage of the usual exception propagation, as long as you make your constructor delegate to another constructor. Such a design can obviate the need for various "init"-functions that used to be popular whenever it wasn't desired to put too much work into a regular constructor.

The specific language that defines the behaviour you're seeing is:

[C++11: 15.2/2]: [..] Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object’s destructor will be invoked. [..]