I am very confused about the const version and non-const version member function like below:
value_type& top() { return this.item }
const value_type& top() const { return this.item }
What is the difference between these two functions? In what situation would they be used?
The const-qualified member function will be called if the member function is called on an object that is const-qualified.
The non-const-qualified member function will be called if the member function is called on an object that is not const-qualified.
For example:
MyStack s;
s.top(); // calls non-const member function
const MyStack t;
t.top(); // calls const member function
Note that the same rules apply when calling a member function on a reference to an object or through a pointer to an object: if the pointer or reference is to a const object, the const member function will be called; otherwise the non-const member function will be called.
In short, they're used to add 'const correctness' to your program.
value_type& top() { return this.item }
This is used to provide mutable access to item
. It is used so you can modify the element in the container.
For example:
c.top().set_property(5); // OK - sets a property of 'item'
cout << c.top().get_property(); // OK - gets a property of 'item'
One common example for this pattern is returning mutable access to an element with vector::operator[int index]
.
std::vector<int> v(5);
v[0] = 1; // Returns operator[] returns int&.
On the other hand:
const value_type& top() const { return this.item }
This is used to provide const
access to item
. It's more restrictive than the previous version - but it has one advantage - you can call it on a const object.
void Foo(const Container &c) {
c.top(); // Since 'c' is const, you cannot modify it... so the const top is called.
c.top().set_property(5); // compile error can't modify const 'item'.
c.top().get_property(); // OK, const access on 'item'.
}
To follow the vector example:
const std::vector<int> v(5, 2);
v[0] = 5; // compile error, can't mutate a const vector.
std::cout << v[1]; // OK, const access to the vector.
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