C++中在类中重载输出运算符时遇到error: declaration of ‘class T‘的问题的解决

一、问题代码及报错提示：

​
#include <iostream>

using namespace std;

template<class T>
class Person{
public:
    template<class T>
    friend ostream& operator<<(ostream& out,Person<T> &p);
public:
    //重载输出运算符
    Person(T age,T id);
    void Show();
private:
    T mAge;
    T mID;
};
template<class T>
Person<T>::Person(T age,T id)
{
    this->mAge = age;
    this->mID = id;
}
template<class T>
void Person<T>::Show()
{
    cout<<"mAge:"<<this->mAge<<" "<<"mID"<<this->mID<<endl;
}
template<class T>
ostream& operator<<(ostream& out,Person<T> &p)
{
    out<<"mAge:"<<p.mAge<<" "<<"mID"<<p.mID<<endl;
    return out;
}

void test01()
{
    Person<int> p(20,10);
   // p.Show();
    cout<<p;

}

int main(int argc, char *argv[])
{
    test01();
    cout << "Hello World!" << endl;
    return 0;
}

​

 二、原因剖析

因为代码中用到类模板template<class T>而在类内声明的友元函数也用到了<T>,此时友元函数是依赖于类的实现的，所以编译器会报错

对此我们有几种解决方案：

1.改变友元与类模板的对应关系为多对多（若类模板使用的是template<class T>，友元函数就使用template<class U>来实现），代码如下：

#include <iostream>

using namespace std;

template<class T>
class Person{
public:
    template<class U>
    friend ostream& operator<<(ostream& out,Person<U> &p);
public:
    //重载输出运算符
    Person(T age,T id);
    void Show();
private:
    T mAge;
    T mID;
};
template<class T>
Person<T>::Person(T age,T id)
{
    this->mAge = age;
    this->mID = id;
}
template<class T>
void Person<T>::Show()
{
    cout<<"mAge:"<<this->mAge<<" "<<"mID"<<this->mID<<endl;
}
template<class U>
ostream& operator<<(ostream& out,Person<U> &p)
{
    out<<"mAge:"<<p.mAge<<" "<<"mID"<<p.mID<<endl;
    return out;
}

void test01()
{
    Person<int> p(20,10);
   // p.Show();
    cout<<p;

}

int main(int argc, char *argv[])
{
    test01();
    cout << "Hello World!" << endl;
    return 0;
}

2.改变友元与类模板的关系为一对一（朋友关系被限定在相同类型的实例化）代码如下：

#include <iostream>

using namespace std;
//前置声明
template<class T>
class Person;
template<class T>
ostream& operator<<(ostream& out,Person<T> &p);

template<class T>
class Person{
public:
    friend ostream& operator<<<T>(ostream& out,Person<T> &p);
public:
    Person(T age,T id);
    void Show();
private:
    T mAge;
    T mID;
};
template<class T>
Person<T>::Person(T age,T id)
{
    this->mAge = age;
    this->mID = id;
}
template<class T>
void Person<T>::Show()
{
    cout<<"mAge:"<<this->mAge<<" "<<"mID"<<this->mID<<endl;
}
template<class T>
ostream& operator<<(ostream& out,Person<T> &p)
{
    out<<"mAge:"<<p.mAge<<" "<<"mID"<<p.mID<<endl;
    return out;
}

void test01()
{
    Person<int> p(20,10);
   // p.Show();
    cout<<p;

}

int main(int argc, char *argv[])
{
    test01();
    cout << "Hello World!" << endl;
    return 0;
}

3.使用普通友元函数PrintPerson来实现，代码如下：

#include <iostream>

using namespace std;

template<class T>
class Person{
public:
    //普通友元函数的声明
    template<class U>
    friend void PrintPerson(Person<U>& p);
public:
    Person(T age,T id);
    void Show();
private:
    T mAge;
    T mID;
};
template<class T>
Person<T>::Person(T age,T id)
{
    this->mAge = age;
    this->mID = id;
}
template<class T>
void Person<T>::Show()
{
    cout<<"mAge:"<<this->mAge<<" "<<"mID"<<this->mID<<endl;
}
//普通友元函数的实现
template<class U>
void PrintPerson(Person<U>& p)
{
    cout<<"mAge:"<<p.mAge<<" "<<"mID"<<p.mID<<endl;
}
void test01()
{
    Person<int> p(20,10);
   // p.Show();
    //cout<<p;
    PrintPerson(p);

}

int main(int argc, char *argv[])
{
    test01();
    cout << "Hello World!" << endl;
    return 0;
}