Why can't I return this vector of vectors?

Question

I have a class that declares a vector of vectors of type char as the following and contains a method to read half of it and return the resulting vector.

class Mapa { private: vector<vector<char> > vec; public: //Constructor Mapa():vec(0,vector<char>(0)){}; Mapa(int row, int col):vec(row, vector<char>(col)){}; virtual ~Mapa(){}; } const Mapa& just_Half() { Mapa temp(vec.size(),vec[0].size()); int a = vec.size()*vec[0].size()/2; for (int i = 0; i < vec.size() && a>0; i++, a--) for (int j = 0; j < vec[i].size() && a>0; j++, a--){ temp.vec[i][j]=vec[i][j]; } return temp; }

in main I do:

Mapa Mapa1(3, 10); Mapa Mapa2; Mapa2=Mapa1.just_Half();

My question is why is Map2 just and empty vector at the end of this? Why doesn't it receive temp?

Thanks for any help.

The issue is not related to Visual C++, hence I removed that tag. You're returning a reference to a local variable. The issue has been described in many questions such as stackoverflow.com/q/4643713 and stackoverflow.com/q/2744264 — dyp
– dyp, Commented Apr 27, 2015 at 19:23
Oops, I wanted to say "The issue is not specific to Visual C++". — dyp
– dyp, Commented Apr 27, 2015 at 19:29
Thanks for the indication. Having a hard time with pointers and references. — hadroque
– hadroque, Commented Apr 27, 2015 at 21:16

Community · Accepted Answer · 2017-05-23 11:50:48Z

The issue is that your return type is a reference to a Mapa. The Mapa (named temp inside the function) you're returning is destructed when the function goes out of scope, so you're returning a destructed Mapa, which is undefined.

The solution is simple, return by value instead of by reference:

const Mapa just_Half() { Mapa temp(vec.size(),vec[0].size()); int a = vec.size()*vec[0].size()/2; for (int i = 0; i < vec.size() && a>0; i++, a--) for (int j = 0; j < vec[i].size() && a>0; j++, a--){ temp.vec[i][j]=vec[i][j]; } return temp; }

If you're concerned that this does an unnecessary copy, don't worry. Compilers will almost always optimize that copy operation away with return value optimization.

Yes indeed. But unfortunately I am stuck to using the method as I posted since its an assignment... I kind of sorted it out by using a ' static Mapa temp'
If I were in your position, I would bring this up with your instructor. Your solution absolutely should not have to be to use a statically allocated Mapa object (such that it isn't destructed on function exit).
Yes. Will do since it also doesn't make much sense to me having to use such a solution... Thanks

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