C++ 输入输出流运算符重载为什么用友元函数

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class A {
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public:
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    bool operator==(const A& rhs) const; // 等价于 bool A::operator==(const A* this, const A& rhs)
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};
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a == b; // 实际调用 a.operator==(b), 即 operator==(&a, b)

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class Person {
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private:
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    std::string name;
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public:
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    friend std::ostream& operator<<(std::ostream&, const Person&);
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};
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std::ostream& operator<<(std::ostream& os, const Person& p) {
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    os << p.name;
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    return os;
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}