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#include <iostream>
 
class foo {
public:
   foo() : i(1), j(2) {}
 
   int i; 
   int j; 
   bool operator<(const foo& val) const {
     std::cout << "memba\n";
     return i<val.i;
   }
};
 
bool operator<(const foo& lhs, const foo& rhs) {
   std::cout << "free\n";
   return lhs.j < rhs.j;
}
 
int main()
{
    foo f1, f2;
    bool b1 = f1 < f2;
 
    foo const f3;
    bool b2 = f3 < f2;
 
    return b1 + b2;
}
 

prog.cpp: In function 'int main()':
prog.cpp:23:20: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
prog.cpp:15:6: note: candidate 1: bool operator<(const foo&, const foo&)
prog.cpp:9:9: note: candidate 2: bool foo::operator<(const foo&) const
prog.cpp:26:20: error: ambiguous overload for 'operator<' in 'f3 < f2'
prog.cpp:9:9: note: candidates are: bool foo::operator<(const foo&) const
prog.cpp:15:6: note:                 bool operator<(const foo&, const foo&)

Standard output is empty