"no match for 'operator<<' in 'std::operator<<'" error when compiling in Linux, VS doesn't present this error

Question

For some reason my code works in Visual Studio but not in the Linux compiler and gives me an error in Linux saying

test_main.cpp:65: error: no match for 'operator<<' in 'std::operator<<'

With tons of lines inside of [] my overloading code

String String::operator + (const String & s) const {
String temp;
temp.head = ListNode::concat(head,s.head);
return temp;
}

my concat code

String::ListNode * String::ListNode::concat(ListNode * L1, ListNode * L2)
{
return L1 == NULL ? copy(L2): new ListNode(L1->info, concat(L1->next, L2));
}

code testing it

String firstString("First");
String secondString("Second");
cout << "+: " << firstString + secondString << endl;

declaration

ostream & operator << (ostream & out, String & l);

Body

ostream & operator << (ostream & out, String & l)
{
l.print(out);
return out;
}

Print method

void String::print(ostream & out)
{
    for (ListNode * p = head; p != nullptr; p = p->next)
        out << p->info;
}

In my Visual Studio 2015 environment this print FirstSecond and doesn't give an error like in Linux and I have no idea why

Answer 1

The problem is with the output operator:

ostream & operator << (ostream & out, String & l);

The result of the operation firstString + secondString is a temporary object, and non-constant references can't bind to temporary object.

You need to change your function to take a reference to a constant object, e.g.

ostream & operator << (ostream & out, String const & l);
//                                           ^^^^^
//                              Note use of `const` here