CODEWITHSUNDEEP
c++c++11overloadingcopy-constructor
Humam Helfawi
Humam Helfawi

Reputation: 20264

Add copy constructor to a class

Is it possible to add a copy constructor (or in other words to overload it) for class which I did not write?

For example, I am using some library that have a Point class. I want to add a copy constructor to it. I can not and do not want to edit it.

Imaginary syntax:

cv::Point cv::Point::Point(const AnotherPoint& p){
    return cv::Point(p.x,p.y);
}

P.S I did not write AnotherPoint also.

EDIT -Problem Background-:

All my problem that I want to copy std::vector<cv::Point> to another std::vector<AnotherPoint> using the standard function std::copy. So I was seraching for a way to overload the copy constructor to make it.

Upvotes: 1

Views: 125

Answers (2)

Paolo M
Paolo M

Reputation: 12757

That is not possible.

You can do the opposite, though, i.e. define an implicit conversion operator from your type (AnotherPoint) to the the other one (cv::Point).

This way, you will be able to use an object of type AnotherPoint everywhere a cv::Point is expected.

If you don't have control over AnotherPoint too, I guess the only way to go is to define a standalone function that create a cv::Point from a AnotherPoint.

After OP EDIT:

Eventually, to transform a vector<cv::Point> into a vector<AnotherPoint> you can use the standalone function mentioned above as the unary_op of std::transform, as @TartanLlama suggested.

Upvotes: 1

TartanLlama
TartanLlama

Reputation: 65620

You can't add constructors to a type after its definition.

A simple way to copy a std::vector<cv::Point> to a std::vector<AnotherPoint> would be to use std::transform:

std::vector<cv::Point> cvPoints;
//cvPoints filled
std::vector<AnotherPoint> otherPoints;
otherPoints.reserve(cvPoints.size()); //avoid unnecessary allocations
std::transform(std::begin(cvPoints), std::end(cvPoints),
               std::back_inserter(otherPoints), 
               [](const cv::Point& p){ return AnotherPoint{p.x, p.y}; });

Upvotes: 5

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