Question

I have a function that creates a 2D vector

void generate(int n)
{
   vector< vector<int> > V (n, vector<int>(1 << n , 0 ));
   .......

}//n is used to determine the size of vector

Now, I need to return the created vector to use it in another function .If I did

return V ;

it will be wrong because V is a local variable but I can’t define V outside the function because this functions defines the size of V . What should I do ?

Answer 1

You can return V with no issues – it will return a copy of the local variable. Issues only arise when you return a reference or pointer to a variable with local scope; when the function ends, the local variable falls out of scope and is destroyed and the reference/pointer is no longer valid.

Alternatively, you can accept a reference to a vector as your argument, write to it and return void:

void generate(int n, std::vector< std::vector<int> >& vec) {
    vec.resize(n, std::vector<int>(1 << n, 0));
}

int main() {
    std::vector< std::vector<int> > v;
    generate(10, v);
}

This is faster than returning a copy of the local member, which can be expensive for large objects such as multi-dimensional vectors.