C++ getter temporary vs const reference

Question

I have some class that stores vector. And I have getter for it

class Data{
  private:
    std::vector<Point*> points;

  public:
    inline std::vector<Point*> getPoints(){
      return points;
     }

}

When I call getPoints() will it return temporary object? Or it will just copy it?
Or I should return const reference, instead?

UPDATE

And what if I have another class that hold my Data, which also have a getter:

class Controller{
private:
  Data *data;

public:
  inline std::vector<Point*> getPoints(){
     return data->getPoints();
  }
}

Should both of them be const reference, if I want them not to be copied?

possible duplicate of [Should accessors return values or constant references?](http://stackoverflow.com/questions/4613823/should-accessors-return-values-or-constant-references) — fredoverflow, Apr 13 '14 at 13:27

score 2 · Accepted Answer · answered Apr 13 '14 at 12:34

2

Your method makes a copy of points. Modifications to the vector itself will not be propagated to the points inside your class. You could return a const reference instead to avoid copying. The caller would be able to copy if he wants, but he would be in control.

Note that the copy that is made when returning a vector is shallow. This means that the individual points are exposed: if the caller makes modifications to points (as opposed to making modifications to the vector) these modifications will propagate to the points pointed to by the vector inside your class.

Protecting the content of Points is a lot harder - it would require you to do manual copying. It would also require your caller to make manual freeing. In general, though, you would be better off using smart pointers (std::unique_ptr or std::shared_ptr) for easier resource management.

answered Apr 13 '14 at 12:34

Sergey Kalinichenko

714,442
84
1,110
1,523

@tower120 Correct. Had they been values, they would be copied as well. – Sergey Kalinichenko Apr 13 '14 at 12:38
please, look my updated question. As I understand, current implementation will copy my "points" twice. – tower120 Apr 13 '14 at 12:43
@tower120 There will be only one copy of the vector - the compiler is smart enough to optimize away the second copy. – Sergey Kalinichenko Apr 13 '14 at 12:55
By the way - why NVRO not happens here? – tower120 Apr 13 '14 at 19:02
1

@tower120 NVRO optimizes away the constructor call of the vector that you called `pa` in your example, proceeding straight to populating it with the data returned from `getPoints` member function. It does not eliminate copying from `points` to `pa`, but it makes that copy faster. – Sergey Kalinichenko Apr 13 '14 at 19:56

score 0 · Answer 2 · answered Apr 13 '14 at 12:30

0

It will copy your original vector.

Return a const reference if you don't want to have it that way.

answered Apr 13 '14 at 12:30

erenon

18,838
2
61
93

C++ getter temporary vs const reference

2 Answers2