Argument passing of template class as an interface

Question

Here is my scenario:

class Container {};

class Papa {};

class Child: public Papa {};

void myFunction(Container<Papa> container);

Now I want to pass two different parameters to myFunction().

int main() 
{
    Container<Papa> papaContainer;
    Container<Child> childContainer;
    myFunction(papaContainer);
    myFunction(childContainer);  // this line is not working 
}

apparently I cannot do this.

no suitable user-defined conversion from "Container<Child>" to "Container<Papa>"

What would be the solution for this case? How Container<Papa> can be used as interface here?

It's because templates are not covariant. See here: https://stackoverflow.com/a/2206549/745468. One solution could be to make your myFunction also a template. — SMeznaric, Oct 10 '21 at 20:59
@SMeznaric thanks, template covariance was the term I was looking for. — Aryan Firouzian, Oct 10 '21 at 21:12

score -1 · Answer 1 · answered Oct 10 '21 at 19:53

-1

This seems similar to this question here: No Suitable User Defined Conversion although that one is using numeric types. Is there anything in myFunction, or that myFunction returns that would need to differentiate Child from Papa?

answered Oct 10 '21 at 19:53

CyanMagentaYellowBlack

74
7

No it is not similar to that question. Completely different question. – Aryan Firouzian Oct 10 '21 at 19:58

Argument passing of template class as an interface

1 Answers1