Why catch an exception as reference-to-const?

Question

I've heard and read many times that's better to catch an exception as reference-to-const rather than as reference. Why is

try {
    // stuff
} catch (const std::exception& e) {
    // stuff
}

better than

try {
    // stuff
} catch (std::exception& e) {
    // stuff
}

Answer 1

It tells the compiler that you won't be calling any function which modify the exception, which may help to optimize the code. Probably doesn't make much of a difference, but the cost of doing it is very small too.

Answer 2

For the same reason you use a const.

Answer 3

are you going to modify the exception? if not, it may as well be const. same reason you SHOULD use const anywhere else (I say SHOULD because it doesn't really make that much difference on the surface, might help compilers, and also help coders use your code properly and not do stuff they shouldn't)

exception handlers, may be platform specific, and may put exceptions in funny places because they aren't expecting them to change?

Answer 4

You need:

• a reference so you can access the exception polymorphically
• a const to increase performance, and tell the compiler you're not going to modify the object

The latter is not as much important as the former, but the only real reason to drop const would be to signal that you want to do changes to the exception (usually useful only if you want to rethrow it with added context into a higher level).