Sorry, I should have been more specific.

C/C++ const has no effect on optimization, unless it's on a global/static object and the compiler can see the original declaration.

I'm not going to look it up in the C++ spec, but in C it's only undefined behavior if the original object was const, and it would make sense for C++ to be the same.

But if you have a const reference to a non-const object, it's legal to use a const_cast to remove the const part. This is the more likely scenario, since most objects aren't const.