c++ - Why using the const keyword before and after method or function name?

Question

I have the following code in my application. Why do we use the const keyword with the return type and after the method name?

const T& data() const { return data_; }

Answer 1

const T& data() const { return data_; }
^^^^^

means it will return a const reference to T (here data_)

Class c;
T& t = c.data()             // Not allowed.
const T& tc = c.data()      // OK.

---

---

const T& data() const { return data_; }
                ^^^^^

means the function will not modify any member variables of the class (unless the member is mutable).

void Class::data() const {
   this->data_ = ...;  // is not allowed here since data() is const (unless 'data_' is mutable)
   this->anything = ... // Not allowed unless the thing is 'mutable'
}