Why cannot c++ compiler find operator<< - c++

Question

Why cannot compiler find operator<<. Where is the compiler looking into to find the definition of operator<< when it encounters the line
cout <<f.some_func()<<endl;
The error:
error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::vector<std::vector<int> >’)
cout <<f.some_func()<<endl;
some notes:....
error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
cout <<f.some_func()<<endl;
#include <iostream>
#include <string>
#include<vector>
using namespace std;
struct Bar
{
int y;
};
class Foo
{
int x;
friend ostream & operator<<(ostream & os, Foo& f)
{
os << "Foo: " << f.x << endl;
return os;
}
friend ostream & operator<<(ostream & os, Bar& b)
{
Foo f;
os << "Bar b.y: " << b.y << endl;
os << "Bar f.x: " << f.x << endl;
return os;
}
friend ostream & operator<<(ostream & os, vector<vector<int> > const& vec ){
os << 5;
return os;
}
public:
Foo(int x = 10):x{x}{}
vector<vector<int> > some_func(){
vector<vector<int> > abc(3, vector<int>(3));
int x = 900;
return abc;
}
//If I do this
void wrapper(){
cout << this->some_func() << endl;
}
};
int main()
{
Bar b { 1 };
Foo f{ 13 };
cout << f << endl;
//cout << b << endl;
cout <<f.some_func()<<endl;
return 0;
}

Answer 1

Functions like this that are defined as friends inside the class have rather unusual name lookup rules. They're free functions, but their names are inside of the class of which they're a friend. Because of this, they're only found via argument-dependent lookup.
For that to work, however, at least one of the arguments has to be something like a reference to an object of the class in which it's defined, or else something defined inside that class.
In your case, the arguments are an iostream and a vector<vector<int>>, neither of which has any relationship to Foo, so the compiler doesn't use argument-dependent lookup to find the function inside of Foo.
The obvious way to make it work is to have the overloads for Foo and Bar inside the definitions of Foo and Bar, and the overload for std::vector<vector<int>> outside any class:
#include <iostream>
#include <string>
#include<vector>
using namespace std;
struct Bar
{
int y;
friend ostream & operator<<(ostream & os, Bar& b)
{
return os << "Bar b.y: " << b.y;
}
};
class Foo
{
int x;
friend ostream & operator<<(ostream & os, Foo& f)
{
return os << "Foo: " << f.x;
}
public:
Foo(int x = 10) : x{ x } {}
vector<vector<int> > some_func() {
vector<vector<int> > abc(3, vector<int>(3));
int x = 900;
return abc;
}
};
ostream & operator<<(ostream & os, vector<vector<int> > vec) {
return os << 5;
}
int main()
{
Bar b{ 1 };
Foo f{ 13 };
cout << f << '\n';
cout << b << '\n';
cout << f.some_func() << '\n';
return 0;
}