CODEWITHSUNDEEP
c++stdvectormember-functionsconst-correctnessfunction-qualifier
jlack
jlack

Reputation: 315

How can you return a non-const reference to an element in a std::vector data member from a const member function?

I'm having a hard time figuring out how to return a non-const reference to an element in a std::vector from a const member function. A simple example of what I'm going for is,

template<class T>
class MyClass
{
 public:
 MyClass : myVec(3)
 {
 }

 T& x() const
 {
    return *(myVec.data())[0]
 }

 std::vector<T> myVec;
}

The behavior I'm going for is that I'd like to be able to do things like the following,

MyClass obj<double>;
obj.x() = 3.3;
assert(obj.x()==3.3)

Eigen gives the same type of behavior, but I've not been able to figure out how to get it to work.

Upvotes: 3

Views: 2323

Answers (2)

songyuanyao
songyuanyao

Reputation: 172924

You could use const_cast, and it'll do the work here, (according to the usage posted). But you won't need it for this case (and for most cases).

You could (and should) add non-const member function overloading for it. Then const member function returns reference to const, non-const member function returns reference to non-const. The appropriate one will be invoked via overload resolution.

const T& x() const
{
   return myVec[0];
}
T& x()
{
   return myVec[0];
}

Upvotes: 11

nate
nate

Reputation: 1871

Standard library containers extend the const-ness of the container to the values themselves. Since your class is constant in x(), so is the vector. You could use const_cast to remove the const from the item in your accessor.

return const_cast <T&> (*(myVec.data())[0]);

In general this is considered bad practice, since const is there to protect you. Bypassing those protections can make the code harder to reason about.

Upvotes: 3

Related Questions