What does const after function header do?
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From the Const Correctness tutorial:
If you have a const object, you don't want to call methods that can change the object, so you need a way of letting the compiler know which methods can be safely called. These methods are called "const functions", and are the only functions that can be called on a const object. Note, by the way, that only member methods make sense as const methods. Remember that in C++, every method of an object receives an implicit this pointer to the object; const methods effectively receive a const this pointer.
It might be worthwhile (spoiler: it is) to read through the whole article if you're new to the concept of constness.
这意味着功能（操作员）不会更改对象。
Effectively makes the "this" pointer a pointer to a const object. It means that members of the object cannot be modified in that method, nor can that method be invoked on a non-const object.
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