重载问题 C++

我似乎遇到了操作员过载的问题。我真的无法弄清楚错误想说什么。这是错误：

Error: no match for 'operator<<' in
'std::cout << s1.Set::operator+(((const Set&)((const Set*)(& s2))))'

这是我的代码：

#include "Set.cpp"
int main(int argc, char *argv[]){
    Set s1(10),s2(6),s3(3),s4;
    cout<<"First set ({x,y,z}): ";
    cin>>s1;    
    cout<<"A: "<<s1<<endl;
    cout<<"Second set: ";
    cin>>s2;
    cout<<"B: "<<s2<<endl;

    cout<<s1+s2<<endl;

}

class Set {
private:
    bool elements[255];
    int capacity; //Yes, an unsigned char, short, or even size_t, would be better
    Set(const bool elements[255], int capacity); //Helpful for immutable types
public:
    Set();
    Set(short capacity);

    friend std::ostream& operator<<(std::ostream &out, Set &set);
    friend std::istream& operator>>(std::istream &in, Set &set);

    int getCapacity() const; //Cardinality of universe. i.e. |Universe| (or just 'capacity')

};
Set::Set(const bool elements[255], int capacity){
    this->capacity = capacity;
    for(int i=0; i<255;i++){
        if(elements[i] == true && i <= capacity){
            this->elements[i] = true;
        }       
        else{
            this->elements[i] = false;
        }           
    }

}
Set::Set(short capacity){   
    this->capacity = capacity;
}
std::ostream& operator<<(std::ostream &out, Set &set) { 
    int capacity = set.getCapacity();
    out<<"{";   
    for(int i=0; i < 255; i++){     
        if(set.elements[i] == true ){
            out<<i<<",";
        }

    }
    out<<"}";
    return out;
}
std::istream& operator>>(std::istream &in, Set &set) {
    bool arr[255];
    int cap=set.getCapacity();  
    char open;
    in>>open;
    if (in.fail() || open!='{') {
        in.setstate(std::ios::failbit);
        return in;
    }
    for (int i=0;i<cap;i++)
        arr[i]=false;
    std::string buff;
    std::getline(in,buff,'}');
    std::stringstream ss(buff);
    std::string field;
    while (true) {
        std::getline(ss,field,',');
        if (ss.fail()) break;
        int el;
        std::stringstream se(field);        
        se>>el;

        if (el>=0&&el<cap){
            arr[el]=true;           
        }
        else{
            arr[el]=false;
        }
    }
    set=Set(arr,cap);
}
Set Set::operator+(const Set &other) const{
    bool arr[255];

    for(int i=0; i<255;i++){
        if(this->elements[i] == true || other.elements[i]==true)
            arr[i] == true;
    }

    int capacity = this->capacity>=other.capacity?this->capacity:other.capacity; 
    return Set(arr,capacity);
}

我重载了 + 和 >> 运算符。执行代码时，它会不会先执行重载的 + 运算符，然后再执行 >> 运算符。

只需要澄清一下。谢谢