CODEWITHSUNDEEP
c++templates
MMS
MMS

Reputation: 57

C++, template variable in non template class

I am using the following template classes:

template <class T>
class Point2D
{
    private:
            T x;
            T y;
...
};

template <class T>
class Point2D;

template <class T>
class Line{
    private:
          Point2D <T> *start;  
          Point2D <T> *start; 
    .... 
};

If I want to create an object Line, it is necessary to write type of the point and type of the Line

int main
{
     Point2DC<double> p1(0,0);
     Point2DC<double> p2(10,10);
     Line<double> l(&p1,&p2);
     ...
}

I find it rather pointless... If points are double, so Line must be also double... Is it possible to templatize only pointers in class Line and do not templatize all class, something like that

template <class T>
class Point2D;

class Line{
    private:
          template <class T>
          Point2D <T> *start;  
          Point2D <T> *start; 
    .... 
};

and use

int main
{
     Point2D<double> p1(0,0);
     Point2D<double> p2(10,10);
     Line l(&p1,&p2);
     ...
}

Upvotes: 4

Views: 1643

Answers (1)

Eclipse
Eclipse

Reputation: 45533

Not directly. You can make a function make_line along the lines of std::make_pair that implicitly figures out the return type based on the input types, but its return type will still be Line<double>. This is useful if you're constructing an anonymous Line for passing into another function.

In C++0X, there's a new use of the auto keyword for declaring an implicitly typed variable, based on the type of the assigned expression.

So this would allow to do something like this (without changing your Point2D or Line classes):

template <class T>
Line<T> make_line(Point2D<T> *p1, Point2D<T> *p2)
{
    return Line<T> (p1, p2);
}

template <class T>
void DoSomethingWithALine(const Line<T> &l)
{
     ....
}

int main
{
     Point2DC<double> p1(0,0);
     Point2DC<double> p2(10,10);
     // C++0X only:
     auto l = make_line(&p1,&p2);

     // Current C++:
     DoSomethingWithALine(make_line(&p1, &p2));
     ...
}

Upvotes: 6

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