Redundant "const" harms performance ⏱

The correct using of const is a big topic in C++. The coolest explanation of this topic is probably there: Const Correctness, C++ FAQ, but it doesn’t tell about one disadvantage of the “redundant” const.

Let’s say we have a structure that represents an API object. The object is immutable, so at first glance it makes sense to declare all field const:

struct Widget {
    const std::size_t radius;
    const std::vector<Event> events;
    const Element element;
};

But this is bad when objects of this type are moved, are hold in containers, and so on. The fact is that const-fields are not movable, therefore when we do Widget w2{std::move(w1)} the fields events and element are copied.

(godbolt example with logs)

Const fields are often not necessary – it is enough to have a const object (or a pointer/reference to a const object):

void Do(const Widget& widget);