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//#include<iostream>
using namespace std;
#include<string>
 
class Expr{};
class NumExpr : public Expr {
        public:
            NumExpr( string v );
            string name();
        private:
            int number;
            friend ostream& operator<<(ostream &s, const NumExpr &num);
    };
 
 NumExpr::NumExpr( string n ) {
        number = 10;
    }
    string NumExpr::name() {
        return "num";
    }
    ostream & operator<<(ostream &s, const NumExpr &num) {
        s << num.number;
        return s;
    }
int main()
{
     //NumExpr obj("hello");
     //cout<<obj;
 
 
     return 0;
}

prog.cpp: In function ‘std::ostream& operator<<(std::ostream&, const NumExpr&)’:
prog.cpp:22: error: no match for ‘operator<<’ in ‘s << num->NumExpr::number’
prog.cpp:21: note: candidates are: std::ostream& operator<<(std::ostream&, const NumExpr&)

Standard output is empty