C + + friend learning notes __c++

Friends can be divided into three kinds:

1. Friend function;

2. Friend class;

3. Friend member function;

The benefit of friends, by using the friend function, you can give the function the same access as the class member function, which is a non-class member function that can access the private members of the class.

Because a friend function is not a member function of a class, it is not possible to invoke a member function in the same way as a class object (class member).

Creation of friend functions

Template<class t> class Matrix {friend ostream& operator<< (ostream& os, const matrix<t>& m {for (int i = 0; i < m.therows. i++) {for (int j = 0; J < M.thecolumns; J +) {OS << M.E
			Lement[i*m.thecolumns + j]<< "";
		Os << Endl;
		Os << "Output Matrix finished" << Endl;
		
	return OS;
	}; Friend istream& operator >> (istream& os, matrix<t>& m) {size_t number = M.thecolumns * M.ther
		oWS
		for (int i = 0; i < number; i++) {OS >> m.element[i];
		cout << "Input fanished" << Endl;
	return OS;
};
	Public:matrix (int therows = 0, int thecolumns = 0);
	Matrix (const matrix<t>&);
	~matrix () {delete[] element;};
	int rows () const {return therows;};
	int cols () const {return thecolumns;};
	t& operator () (int i, int j) const;
	matrix<t>& operator= (const matrix<t>&);
	Matrix<t> operator+ () const; MatriX<t> operator+ (const matrix<t>&) const;
	Matrix<t> operator-() const;
	Matrix<t> operator-(const matrix<t>&) const;
	Matrix<t> operator* (const matrix<t>&) const;
	matrix<t>& operator+= (const t&);
	void Input ();
	void Output ();
void Transport ();
	Private:int therows;
	int thecolumns;
t* element; };

Friend ostream& operator<< (ostream& os, const matrix<t>& m);

Is the way to create a friend function;

function is created, you need to define the

The definition of a friend function requires neither the class header nor the keyword friend;

<< is the most common friend function, overload << operator

In peacetime programming we often use cout, this is a Ostream object, it is intelligent, can recognize the basic types of C + +.

As a friend function overload << is to achieve the output of class object to bring convenience;

Today I fell into a hole in the friend, in theory, it is okay to define a friend function outside of the class, but the way I define it is that the program compiles unsuccessfully and always reports illegal use of explicit templates

Template<class t>
std::ostream& operator<< <T> (ostream& os, const matrix<t>& m)
{for

	(int i = 0; i < m.therows. i++)
	{for
		(int j = 0; J < M.thecolumns; J +)
		{
			OS <& Lt M.element[i*m.thecolumns + j] << "";
		}
		Os << Endl;
	}
	Os << "Output Matrix finished" << Endl;
	return OS;

Looking for a long time on the internet did not solve the problem, in the groping, I directly define the friend function within the class, and then the smooth running of the program, as for the truth, I still need to grope for groping.