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#include <iostream>
using namespace std;
 
struct foo {
	int x;
	int y;
	int z;
};
 
int& operator [](foo& lhs, const std::size_t rhs) {
	switch(rhs) {
	case 0U:
	    return lhs.x;
	case 1U:
		return lhs.y;
	case 2U:
		return lhs.z;
	default:
		return *(&(lhs.z) + rhs - 2U);
	}
}
 
int main() {
	foo my_foo{};
 
	cout << foo[0U] << endl;
}

prog.cpp:10:49: error: ‘int& operator[](foo&, std::size_t)’ must be a nonstatic member function
 int& operator [](foo& lhs, const std::size_t rhs) {
                                                 ^
prog.cpp: In function ‘int main()’:
prog.cpp:26:13: error: expected primary-expression before ‘[’ token
  cout << foo[0U] << endl;
             ^

Standard output is empty