【c++】cout重载能不能写成成员函数，若能，写出函数原型，若不能，说明原因

// cout重载能不能写成成员函数，若能，写出函数原型，若不能，说明原因

#include <iostream>
using namespace std;

// cout做友元
class A;
ostream& operator<<(ostream &out, const A &a);

class A
{
	friend ostream& operator<<(ostream &out, const A &a);
public:
	A(int n = 0):m(n)
	{}
private:
	int m;
};

ostream& operator<<(ostream &out, const A &a)
{
	out << a.m;
	return out;
}

int main()
{
	A a;
	cout << a << endl;
	return 0;
}

// cout并不是不可以做成员函数，但是它的输出形式并不符合大家的习惯，所以还是不要把它写到成员函数里比较好
#include <iostream>
using namespace std;

class A
{
public:
	A(int n = 0) :m(n)
	{}
	// cout做成员函数
	ostream& operator<<(ostream &out)
	{
		out << m;
		return out;
	}
private:
	int m;
};

int main()
{
	A a;
	//注意输出格式
	a << cout << endl;
	return 0;
}

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