Question

We know that the exception class has two derived classes: logic_error and runtime_error.

logic_error has four derived classes: domain_error, invalid_argument, length_error and out_of_range.

runtime_error has three derived classes: range_error, overflow_error and underflow_error.

While some of them are self-explanatory, like overflow_error and underflow_error, some are not that clear, especially range_error, both MSDN and cplusplus just say “to report a range error”, which is close to saying nothing, how it is different out_of_range and domain_error???

Another question is when I throw an exception, which one should I choose? For example, In reverse_string(char* s), which exception to throw when s is NULL? In float calc_ellipse_area(float a, float b), which to throw when a or b is <=0? Which to throw when a == b (strictly speaking, circle is not an ellipse!)?

Finally, practically speaking, does it really matter if I throw an exception which is not correctly categorized?

Answer 1

For example, In reverse_string(char* s), which exception to throw when s is NULL?

In float calc_ellipse_area(float a, float b), which to throw when a or b is <=0? Which to throw when a == b (strictly speaking, circle is not an ellipse!)?

For both of these, use std::invalid_argument.

Or you can define your own exception called null_argument deriving from std::logic_error (or from std::invalid_argument), and use it for NULL argument.

The point is that if none of the standard exception class applies to your situation, or you want more specific exception, then define one deriving from existing classes.

For example, if you want to throw exception when you encounter an invalid index, then either you can use std::out_of_range or define more specific class called index_out_of_range deriving from std::out_of_range.

does it really matter if I throw an exception which is not correctly categorized?

Yes, it does matter. For example, it increases readability of your code. If you throw std::logic_error when an invalid index is encountered, then well, it doesn’t add much to the readability, but instead of that if you throw std::out_of_range , then it greatly increases the readability. And if you throw index_out_of_range, it increases even more, as it’s more specific.