binary '<<': no operator found which takes a right-hand operand of type 'const studentinfo

[masterinex]

when i run this code , i am getting

Severity Code Description Project File Line Suppression State
Error C2679 binary '<<': no operator found which takes a right-hand operand of type 'const studentinfo' (or there is no acceptable conversion) studentrecord c:\users\admin\desktop\visualstudio\examreview\studentrecord\source.cpp 65

i thought i have taken care of it, because i have overloaded the << operator


ostream& operator <<(ostream& os, studentinfo & c)
{
os << c.name << " " << c.grade ;
return os;

}

 
# include <iostream>
# include <algorithm>
# include <vector>
# include <list>
# include <string>
# include <set>


using namespace std;

struct studentinfo {
	string name;
	int grade;

	bool operator<(const studentinfo& other) const { return name < other.name; }

};


class markinfo {

public:

	vector<studentinfo> pass;
	vector <studentinfo> fail;

};


markinfo passfail( const vector<studentinfo>  & s)
{
	markinfo m;

	for(auto x:s)

	if (x.grade >= 50)
	{
		m.pass.push_back(x); 
	}
	else
	{
		m.fail.push_back(x);
	}

	return m; 

}


ostream& operator <<(ostream& os, studentinfo & c)
{
	os << c.name << " " << c.grade ;
	return os;

}



template<typename T>
void print(T& c)
{
	cout << "\nHere is the List: \n";
	for (auto& x : c)			//& pass by ref is required if process modifies the element
		cout << x << "  ";
	cout << "\n" << endl;
}




bool cmp_names(const studentinfo & lhs, const studentinfo & rhs)
{
	return lhs.name < rhs.name; 
}




int main()
{

	vector <studentinfo> s;

	studentinfo sinfo;
	sinfo.name = "gg";
	sinfo.grade = 44;

	studentinfo sinfo2;
	sinfo2.name = "aa";
	sinfo2.grade = 77;

	s.push_back(sinfo);
	s.push_back(sinfo2);

	sort(s.begin(), s.end(), cmp_names);

	markinfo m;

	m = passfail(s);
	cout << " test " << endl; 


	list <studentinfo> l;

	std::list<studentinfo> lst{ s.begin(), s.end() };

	set<studentinfo> ss(s.begin(), s.end()); 

	print(lst); 

	print(ss); 


	for (auto& x : s)			//& pass by ref is required if process modifies the element
		cout << x << endl;


	
	//l.insert((s.begin(), s.end()));


	//ist <studentinfo> ll = ;

	/*
	for (auto x : m.pass)
	{
		cout << x.name << x.grade << endl;
	}*/

	
	for (auto x : s)
	{
		cout << x.name << x.grade << endl;
	}
	
	system("pause");
	

}

[Peter87]

Use reference to const.

ostream& operator <<(ostream& os, const studentinfo& c)

[masterinex]

why do i need to pass it as a const in:

ostream& operator <<(ostream& os, const studentinfo& c

[Enoizat]

why do i need to pass it as a const

Because the compiler says:

In instantiation of 'void print(T&) [with T = std::set<studentinfo>]':
112:10: required from here 
65:8: error: no match for 'operator<<' (operand types are 
             'std::ostream {aka std::basic_ostream<char>}' and 'const studentinfo')

Your code at line 112 states:
print(ss);
which causes a call to:

template<typename T>
void print(T& c)
{
	cout << "\nHere is the List: \n";
	for (auto& x : c)element
		cout << x << "  ";
	cout << "\n" << endl;
}

Which is a “range-based for loop”
http://en.cppreference.com/w/cpp/language/range-for
and so is turned into:

{

    auto && __range = range_expression ;
    auto __begin = begin_expr ;
    auto __end = end_expr ;
    for ( ; __begin != __end; ++__begin) {

        range_declaration = *__begin;
        loop_statement

    }

} 

Since std::set is an associative container where the value type is the same as the key type,
http://en.cppreference.com/w/cpp/container/set
because of the following requirements from the standard:
C++14 final_draft (n4140) - 23.2.4.6:

For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.

the requested std::set::iterator, is declared const.

(Errors and Omissions Excepted)