CODEWITHSUNDEEP
c++
user3242640
user3242640

Reputation: 33

Return vector element by reference

I have a class A where I want to return a reference to an element in the vector stored in class A.

Class A

private:
vector<B> v;

public:
B& getElement()
{
      //determine i...
      return v[i];
}

In my main I'm setting a B object to what is return by the getElement() function. However, when I modify it, the vector element isn't being modified

main()

B ele = A.getElement();
//modify B...

What should I be doing differently here?

Upvotes: 3

Views: 2257

Answers (3)

Vlad from Moscow
Vlad from Moscow

Reputation: 311126

Using this statement

B ele = A.getElement();

object ele contains a copy of an element of the vector. You should use a reference to an element of the vector

B &ele = A.getElement();

But take into account that the reference can became invalid if for example the vector will be resized. So it is better to change an element of the vector in one statement with the function call. For example

A.getElement() = SomeNewValue;

Also you could overload operator [] for the class. In this case the code could look as

A a;
a[i] = SomeNewValue;

Upvotes: 0

Michael Burr
Michael Burr

Reputation: 340476

This code:

B ele = A.getElement();
//modify B...

makes a copy of the element returned by reference from A.getElement() into ele.

If you want ele to refer to the element in A then try:

B& ele( A.getElement());
//modify ele...

Upvotes: 2

JaredPar
JaredPar

Reputation: 755457

The getElement method is correctly returning a reference to a B instance but the local ele is not a reference it is a value. What is essentially happening under the hood here is the following 

// Copy constructor B::B(B& other) is being called 
B ele = B(A.getElement());

If you want to ele to be a reference then you need to declare it as such

B& ele = A.getElement();

Upvotes: 6

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