CODEWITHSUNDEEP
c++casting
black sheep
black sheep

Reputation: 399

Understanding const_cast in C++

I am trying to to understand const_cast.

In the example below, items_ is a private member variable.

The method getItems() is a const method, which means that it cannot modify the member variable items_.

getItems() returns a const vector reference which means that you cannot modify the member variable items_.

My question is, is const_cast necessary here? I don't think so as I don't need to cast away constness?

#include <iostream>

class test {  
  std::vector<Item> items_;
public:
  const std::vector<Item>& getItems() const;
};

const std::vector<mv::Item>& mv::Workloads::getItems() const
{
  return const_cast<std::vector<Item>&>(items_);
}

Upvotes: 1

Views: 230

Answers (2)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122458

It is not only unnecessary but it is plain wrong in that place.

Consider what happens when you remove the const from the return type:

/*const*/ std::vector<mv::Item>& mv::Workloads::getItems() const
{
return const_cast<std::vector<Item>&>(items_);
}

Now the method returns a non-const reference to a const object! Note that the method itself is declared as const, hence this is const in the context of this method and also items_ is const. Using the reference returned by that method would lead to undefined behaviour when the method is called on a const test instance.

In that sense, the only effect of the const_cast here is to potentially silence an important compiler error should you ever decide to change the return type.

On the other hand, in the method as is (ie returning a const reference) there is no reason for the cast. items_ is const and a const reference to it is returned.

const_cast is useful eg to avoid code duplication for const and non-const methods when you do know for sure that the object you cast const away really is not const, see here for details.

PS: in case you actually do want to return a mutable reference to a member from a const method, you would declare the member mutable. Also then no const_cast would be required. Though, mutable should be used with care and usually isnt needed in the first place.

Upvotes: 5

Lorlin
Lorlin

Reputation: 884

It is not neccessary, simply return the _items.

Const member functions see the the object itself (*this) as const, so every data member is seen as const, from which building and returning const reference is allowed.

#include <iostream>
using namespace std;
class test {

std::vector<Item> items_;
public:
const std::vector<Item>& getItems() const;
};
const std::vector<mv::Item>& mv::Workloads::getItems() const
{
return items_;
}

Upvotes: 3

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