Question

This is my template matrix class:

template<typename T>
class Matrix
{
public:
....
Matrix<T> operator / (const T &num);
}

However, in my Pixel class, I didn’t define the Pixel/Pixel operator at all!

Why in this case, the compiler still compiles?

Pixel class

#ifndef MYRGB_H
#define MYRGB_H

#include <iostream>
using namespace std;

class Pixel
{
public:
    // Constructors
    Pixel();
    Pixel(const int r, const int g, const int b);
    Pixel(const Pixel &value);
    ~Pixel();

    // Assignment operator
    const Pixel& operator = (const Pixel &value);

    // Logical operator
    bool operator == (const Pixel &value);
    bool operator != (const Pixel &value);

    // Calculation operators
    Pixel operator + (const Pixel &value);
    Pixel operator - (const Pixel &value);
    Pixel operator * (const Pixel &value);
    Pixel operator * (const int &num);
    Pixel operator / (const int &num);

    // IO-stream operators
    friend istream &operator >> (istream& input, Pixel &value);
    friend ostream &operator << (ostream& output, const Pixel &value);

private:
    int red;
    int green;
    int blue;
};

#endif

Answer 1

C++ templates are instantiated at the point you use them, and this happens for the Matrix<T>::operator/(const T&), too. This means the compiler will allow Matrix<Pixel>, unless you ever invoke the division operator.