C++: How to declare private member objects

Question

Possible Duplicate:
How do you use the non-default constructor for a member?

I have the current code:

class ImagePoint {
private:
    int row;
    int col;

public:
    ImagePoint(int row, int col){
        this->row = row;
        this->col = col;
    }

    int get_row(){
        return this->row;
    }

    int get_col(){
        return this->col;
    }
};

And I want to do this:

class TrainingDataPoint{
private:
    ImagePoint point;
public:
    TrainingDataPoint(ImagePoint image_point){
        this->point = image_point;
    }
};

But this wont compile because the line ImagePoint point; requires the ImagePoint class to have an empty constructor. The alternative (from what I have read) says I should use a pointer:

class TrainingDataPoint{
private:
    ImagePoint * point;
public:
    TrainingDataPoint(ImagePoint image_point){
        this->point = &image_point;
    }
};

However, once the constructor has finished running will this pointer point to a cleared up object? If so, do I have to make a copy of the image_point? will this require a copy constructor?

score 10 · Accepted Answer · answered Oct 22 '12 at 15:22

10

You need to use a constructor initializer list:

TrainingDataPoint(const ImagePoint& image_point) : point(image_point){
}

You should prefer this when possible. However, there are situations when you must use it:

members with no default constructors (as you mentioned)
member references
const members

answered Oct 22 '12 at 15:22

Luchian Grigore

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And this will make a copy of image_point and store it in point? – Aly Oct 22 '12 at 15:25
thanks, I will accept answer in 10 mins when SO lets me :). – Aly Oct 22 '12 at 15:26

score 2 · Answer 2 · answered Oct 22 '12 at 15:42

You don't need to know these things since you're not going to use that code, but just for completeness:

once the constructor has finished running will this pointer point to a cleared up object?

Yes, the parameter image_point is destroyed when the constructor exits. So you're right, it would be incorrect to store a pointer to it in the object and try to use it after that.

If so, do I have to make a copy of the image_point?

That would do it, but the reason you're not going to use this code is the question of where you would copy it to.

will this require a copy constructor?

Yes, but ImagePoint already has a copy constructor which the compiler generates automatically for you.

john · Answer 3 · 2012-10-22T15:36:22.540

1

What you have read is wrong. The correct alternative is to use an initialisation list

class TrainingDataPoint{
private:
    ImagePoint point;
public:
    TrainingDataPoint(ImagePoint image_point) : point(image_point){
    }
};

By the way this has nothing to do with private members, you'd get the same issue if they were public.

edited Oct 22 '12 at 15:36

answered Oct 22 '12 at 15:23

john

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score 1 · Answer 4 · answered Oct 22 '12 at 15:25

Just use constructor initializer lists:

class TrainingDataPoint 
{
private:
    ImagePoint point;
public:
    TrainingDataPoint(const ImagePoint &imgpt) 
         : point(imgpt)
    {
        // other code here as necessary. point has already been initialized
    }
};

score 1 · Answer 5 · edited May 28 '13 at 10:50

1

Using the constructor initializer will fix your problem.

TrainingDataPoint(const ImagePoint& image_point) : point(image_point){
}

edited May 28 '13 at 10:50

Taryn

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answered Oct 22 '12 at 17:05

LAP

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C++: How to declare private member objects

5 Answers5