How to return a vector reference from a function?

Question

I am trying to return a vector from a function. But if I return the whole vector, then the copying process would take extra time, so I am trying to return a reference to the vector only. How do I do that in C++ ? here's my current function,

vector<ii>& primeFactors(int n)
    {
        int i;
        vector<ii> factors;
        for(i=0; i<=n && n!=1 ; ++i)
        {
            int cnt = 0;
            while((n%primes[i]) == 0)
            {
                ++cnt;
                n/=primes[i];
            }
            if(cnt!=0)
                factors.push_back({primes[i],cnt});
        }
        if(i == lim)
            factors.push_back({n,1});



        return factors;
    }

and heres my calling in main()

vector<ii> factors = primes.primeFactors(20);

Just return a value. It makes no sense to return a reference in this case. — juanchopanza
– juanchopanza, Commented Apr 14, 2014 at 11:49
@juanchopanza Not in this case, but there are a lot of situations where this makes sense. — user2848971
– user2848971, Commented Apr 14, 2014 at 11:51
What you try is not a good idea, since one should not return references to local variables. — Codor
– Codor, Commented Apr 14, 2014 at 11:53
If you want to do this, you will need to make the vector a reference PARAMETER of the function, and pass a vector to store the result from outside the function. Remember to clear the vector first. This can allow you to reuse the same vector which may be more performant in certain situations. — Neil Kirk
– Neil Kirk, Commented Apr 14, 2014 at 11:59

sh03 · Accepted Answer · 2014-04-14 11:54:00Z

4

But if I return the whole vector, then the copying process would take extra time, so I am trying to return a reference to the vector only.

Don't worry about that. The compiler will optimize the call via RVO (return value optimization) and elide the copies. Just return the vector by value:

std::vector<ii> primeFactors(int n) { … }

answered Apr 14, 2014 at 11:54

sh03

76.5k39 gold badges177 silver badges282 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

TobiMcNamobi · Accepted Answer · 2014-04-14 12:04:50Z

2

As stated by others you should just return the vector.

But if for whatever reason you really want to return a reference you have to be sure that it "lives" after exiting the function. This is not the case with local variables as in your code.

Instead you could wrap the function in a class and have the vector as a member:

class PrimeFactorCalculator
{
public:
    std::vector<ii>& primeFactors(int n)
    {
        // ...

        return m_resultVector;
    }

private:
    std::vector<ii> m_resultVector;
};

answered Apr 14, 2014 at 12:04

TobiMcNamobi

4,8433 gold badges35 silver badges52 bronze badges

Comments

JNevens · Accepted Answer · 2014-04-14 11:55:46Z

0

You can either just return the value of the vector or you can pass the vector by reference as an argument to your function.

answered Apr 14, 2014 at 11:55

JNevens

12.1k9 gold badges51 silver badges73 bronze badges

Collectives™ on Stack Overflow

How to return a vector reference from a function?

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related