CODEWITHSUNDEEP
c++vectoriteration
John
John

Reputation: 1167

Accessing members in a vector of objects as vectors

I have a c++ class that is contained as a vector of objects in another class. 

class Small
{

public:

   int a;
   int b;    
   int c;
};

class Big 
{
public:

   vector< Small > getSmallVec() const { return m_smallVec;}

private:

   vector< Small > m_smallVec;    
};

I'd like to be able to access the members of the Small class as a vector themselves. In other words, I'd like to have a function in Big like

vector<int> getSmallVec_a()

which would return a vector that contains each value of int Small::a in m_smallVec. The only way I can think of doing it would be to iterate over m_smallVec and copy to a vector of int. Is there a way to do this without copying?

Upvotes: 2

Views: 530

Answers (2)

Ali1S232
Ali1S232

Reputation: 3411

there is no way to do that without seperating small components in diffrent vectors since a vector is just working as an array that means when you are creating a vector of smalls there will be 2 blocks space between small::a variables whereas what you what to return is just an array of ints. and by the way it's much more efficient if you delcere you function as vector <int> & getSmall() since if you don't return a refrence it'll cause a copying m_smallVec and returning it as the result, it also makes it impossible to change any value in m_smallVec.

Upvotes: 0

Doug T.
Doug T.

Reputation: 65599

If this is how you expect to access data and performance is important, have you considered using parallel vectors? Instead of 

  std::vector<Small>

In "Big" you actually maintain 3 vectors

  std::vector<int> aVec;
  std::vector<int> bVec;
  std::vector<int> cVec;

All elements at 0 correspond to the 0'th "Small", and you can easily seperate out all the as from the bs.

This comes a lot of annoying bookkeeping that you'd be have to be sure to get right and encapsulate well. Its not a more elegant solution, I usually dislike this kind of thing and would probably question in a code review, but it would probably give you the best performance for the operations you've described.

Upvotes: 2

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