Question

Does the standard have anything to say about an exception that is caught by reference and what happens to attempts to modify it?

Consider the following code:

class my_exception: public std::logic_error
{
public:
    std::vector<std::string> callstack;
};

void MyFunc()
{
    try
    {
        SomethingThatThrows();
    }
    catch (my_exception & e)
    {
        e.callstack.push_back("MyFunc");
        throw;
    }
}

This is a contrived example, I’m not actually attempting something like this. I was just curious what would happen, based on the suggestion in another thread that exceptions should be caught by const reference.

Answer 1

The exception will change.

§15.3[except.handle]/17:

When the handler declares a non-constant object, any changes to that object will not affect the temporary
object that was initialized by execution of the throw-expression.

When the handler declares a reference to
a non-constant object, any changes to the referenced object are changes to the temporary object initialized
when the throw-expression was executed and will have effect should that object be rethrown.

So if my_exception is caught outside of MyFunc, we’ll see the "MyFunc" entry in the callstack (e.g. http://ideone.com/5ytqN)