Cannot bind error while overloading operator

[keh k lenge]

My draw function returns vector<vector<char>> and I overloaded operator<< but this gives me
no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘const std::vector<std::vector<char> >’)
AND
cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’

when I run cout << rect.draw('2', 'w') << endl;

  const virtual vector< vector<char> > draw(char penChar, char fillChar);

   friend ostream& operator<<(ostream& os, vector< vector<char> >& grid) {
			for (vector<char> vec : grid) {
				for (auto ch : vec) {
					os << ch;
				}
				os << "\n";
			}
			return os;
		}

[Peter87]

Non-const references can't bind to temporary objects.

This should work:

friend ostream& operator<<(ostream& os, const vector< vector<char> >& grid) {

Note that you are copying each row of the grid while iterating over it. You probably want to turn vec into a reference to avoid creating unnecessary copies.

for (const vector<char>& vec : grid) {

[keh k lenge]

IDK, its still gives the same error whn I run cout << rect.draw('2', 'w') << endl;

[Enoizat]

The following code, based on your description but with Peter87‘s directions, works fine (apart from a couple of warnings referring to unused Rect::draw() parameters):

#include <iostream>
#include <limits>
#include <vector>

class Rect {
public:
    const virtual std::vector<std::vector<char>> draw(char penChar, char fillChar);
    friend std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<char>>& grid);
};

const std::vector<std::vector<char>> Rect::draw(char penChar, char fillChar)
{
    std::vector<std::vector<char>> myvec = { { 'a', 'b', 'c'},
                                             { 'd', 'e', 'f'} };
    return myvec;
}

std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<char>>& grid)
{
    for(const std::vector<char>& vec : grid) {
        for (const auto ch : vec) {
            os << ch;
        }
        os << '\n';
    }
    return os;
}

void waitForEnter();


int main()
{
    Rect rect;
    std::cout << rect.draw('2', 'w');
    waitForEnter();
    return 0;
}

void waitForEnter()
{
    std::cout << "\nPress ENTER to continue...\n";
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

Output:

abc
def

Press ENTER to continue...

The problem is: are you sure your code does what you want it to do? For example, why have you declared friend an overloaded operator which doesn’t access private members of your class?