Question

I’m getting confused with implicit and explicit declarations. I don’t know why you need to explicitly say or at certain times. For example,

In my main.cpp

#include <iostream>
#include "Point.h"

int main()
{
    Point<int> i(5, 4);
    Point<double> *j = new Point<double> (5.2, 3.3);
    std::cout << i << *j;
    Point<int> k;
    std::cin >> k;
    std::cout << k;
}

for Point<int> k. Why do I have to use the explicit declaration? I get compile errors otherwise. Or do I have it coded incorrectly in my Point.h file?

Point.h:

#ifndef POINT_H
#define POINT_H

#include <iostream>

template <class T>
class Point
{
public:
    Point();
    Point(T xCoordinate, T yCoordinate);

    template <class G>
    friend std::ostream &operator<<(std::ostream &out, const Point<G> &aPoint);

    template <class G>
    friend std::istream &operator>>(std::istream &in, const Point<G> &aPoint);

private:
    T xCoordinate;
    T yCoordinate;
};

template <class T>
Point<T>::Point() : xCoordinate(0), yCoordinate(0)
{}

template <class T>
Point<T>::Point(T xCoordinate, T yCoordinate) : xCoordinate(xCoordinate), yCoordinate(yCoordinate)
{}


template <class G>
std::ostream &operator<<(std::ostream &out, const Point<G> &aPoint)
{
    std::cout << "(" << aPoint.xCoordinate << ", " << aPoint.yCoordinate << ")";
    return out;
}

template <class G>
std::istream &operator>>(std::istream &in, const Point<G> &aPoint)
{
    int x, y;
    std::cout << "Enter x coordinate: ";
    in >> x;
    std::cout << "Enter y coordinate: ";
    in >> y;
    Point<G>(x, y);

    return in;
}

#endif

Answer 1

For a class template, the template arguments must be specified explicitly.

For a function template, the template arguments can be inferred implicitly.

Point is a class, so explicit declaration is required.