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[C++] Handling Exceptions in Constructors

When you use RAII idiom, there are often situations where constructors have to do complex tasks. These complex tasks can sometimes fail, resulting in throwing exceptions. This raises a concern: Is it okay to throw exceptions in constructors?

The first concern is memory leaks. Fortunately, memory leaks do not occur. Variables created on the stack are released through stack unwinding, and if an exception occurs during heap allocation with the new operator, the new operator automatically deallocates the memory and returns nullptr.

The next concern is whether the destructor of the member variables will be called correctly. However, this is also not a problem. When an exception occurs, member variables can be divided into three categories: fully initialized member variables, member variables being initialized, and uninitialized member variables. Fully initialized member variables have had their constructors called and memory allocations completed successfully. In the example code, this applies to the variable b. When an exception occurs, these variables have their destructors called and resources released properly.

Member variables being initialized, like c in the example code, are those in the middle of initialization when an exception occurs. If an exception occurs during member variable initialization, as in the code above, there is only one such variable. However, no member variables are initialized if an exception occurs in the constructor body.

Lastly, uninitialized member variables do not have their constructors called, so their destructors are not called either. However, this is correct behavior since their constructors have not been called yet.

What about inheritance? Will resources allocated by the parent class be released properly? Of course, there is no problem in this case either. When an exception occurs in a constructor, after the destructors of all fully initialized member variables have been called, the parent class's destructors are called to release resources. Thus, executing the code above will produce the following output.

Returning to the initial question: Is it okay to throw exceptions in constructors?

As seen above, there is no problem...... I hope to say. However, one assumption is required: If the class is well-designed, there is no problem with throwing exceptions in constructors.

So, what is a poorly designed class, and what problems can arise in such cases?

In the code above, the constructor of the class B allocates an A object on the heap and explicitly calls delete in the destructor to release it. If no exception occurs in the constructor, there is no problem. When B is allocated, and if an exception occurs, A is allocated, and when B is destroyed, A is destroyed as well.

However, if an exception occurs in B's constructor, a problem arises. When cleaning up B's member variables, the destructor of A is not called because a is merely a pointer to A. Moreover, since B has not been initialized, its destructor is not called, and a becomes a leaked memory. To prevent this, the lifetime of A should be matched to that of B, as the below code.

This article is the translation of my Korean post written in 2017. Looking back at it now, it is a good example of the importance of choosing meaningful variable names. I apologize for using example code with names like A, B, a, and b.

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