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const T& data() const { return data_; }
^^^^^
 
 意味着它将返回一个const引用给T （这里是data_ ） 
 
Class c;
T& t = c.data()             // Not allowed.
const T& tc = c.data()      // OK.
 
 
 
const T& data() const { return data_; }
                ^^^^^
 
 意味着该函数不会修改该类的任何成员变量（除非该成员是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'
}

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'
}