无法解析的外部命令？

#include<iostream>
using namespace std;

template <typename T>
class CComplex {
private:
    T r,i;
public:
    CComplex(T r=0,T i=0) {
        this->r = r;
        this->i = i;
        cout << "Constructor:(" << this->r <<", "<< this->i << ")" << endl;
    }
    CComplex(const CComplex& c) {
        this->r = c.r;
        this->i = c.i;
        cout << "Copy-on:(" << this->r << ", " << this->i << ")" << endl;
    }
    ~CComplex() {
        cout << "Destructor:(" << this->r << ", " << this->i << ")" << endl;
    }

    CComplex operator+(CComplex& c);
    friend ostream& operator<<(ostream& os, CComplex& c);
};
template <class T>
CComplex<T> CComplex<T>::operator+(CComplex& c) {
    CComplex temp;
    temp.r = this->r + c.r;
    temp.i = this->i + c.i;
    return temp;
}

template <class T>
ostream& operator<<(ostream& os,const CComplex<T>& c) {
    os << "(" << c.r << ", " << c.i << ")" ;
    return os;
}

int main(int argc, const char* argv[]) {
    float r1, i1, r2, i2;
    cin >> r1 >> i1 >> r2 >> i2;
    CComplex<float> c1(r1, i1), c2(r2, i2), c3;
    c3 = c1 + c2;
    cout << c1 << endl;
    cout << c2 << endl;
    cout << c3 << endl;
    return 0;

}

#include<iostream>
using namespace std;

template <typename T>
class CComplex {
private:
    T r,i;
public:
    CComplex(T r=0,T i=0) {
        this->r = r;
        this->i = i;
        cout << "Constructor:(" << this->r <<", "<< this->i << ")" << endl;
    }
    CComplex(const CComplex& c) {
        this->r = c.r;
        this->i = c.i;
        cout << "Copy-on:(" << this->r << ", " << this->i << ")" << endl;
    }
    ~CComplex() {
        cout << "Destructor:(" << this->r << ", " << this->i << ")" << endl;
    }

    CComplex operator+(CComplex& c);
    friend ostream& operator <<<> (ostream& os, const CComplex<T>& c);
};

template <typename T>
ostream& operator <<<> (ostream& os, const CComplex<T>& c) {
	os << "(" << c.r << ", " << c.i << ")";
	return os;
}

template <typename T>
CComplex<T> CComplex<T>::operator+(CComplex<T>& c) {
    CComplex temp;
    temp.r = this->r + c.r;
    temp.i = this->i + c.i;
    return temp;
}




int main() {
    float r1, i1, r2, i2;
    cin >> r1 >> i1 >> r2 >> i2;
    CComplex<float> c1(r1, i1), c2(r2, i2), c3;
    c3 = c1 + c2;
    cout << c1 << endl;
    cout << c2 << endl;
    cout << c3 << endl;
    return 0;

}

主要问题是你的友元类申明时，没有加template <typename T>，下面这样也可以

#include<iostream>
using namespace std;

template <typename T>
class CComplex {
private:
    T r,i;
public:
    CComplex(T r=0,T i=0) {
        this->r = r;
        this->i = i;
        cout << "Constructor:(" << this->r <<", "<< this->i << ")" << endl;
    }
    CComplex(const CComplex& c) {
        this->r = c.r;
        this->i = c.i;
        cout << "Copy-on:(" << this->r << ", " << this->i << ")" << endl;
    }
    ~CComplex() {
        cout << "Destructor:(" << this->r << ", " << this->i << ")" << endl;
    }

    CComplex operator+(CComplex& c);

	template <typename T>
    friend ostream& operator << (ostream& os, const CComplex<T>& c);
};

template <typename T>
ostream& operator << (ostream& os, const CComplex<T>& c) {
	os << "(" << c.r << ", " << c.i << ")";
	return os;
}

template <typename T>
CComplex<T> CComplex<T>::operator+(CComplex<T>& c) {
    CComplex temp;
    temp.r = this->r + c.r;
    temp.i = this->i + c.i;
    return temp;
}




int main() {
    float r1, i1, r2, i2;
    cin >> r1 >> i1 >> r2 >> i2;
    CComplex<float> c1(r1, i1), c2(r2, i2), c3;
    c3 = c1 + c2;
    cout << c1 << endl;
    cout << c2 << endl;
    cout << c3 << endl;

    return 0;

}

参考如下方法解决：

https://blog.csdn.net/xiamentingtao/article/details/44858363

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