Subscript operator overloading woes (const related?)

I have the following code:

Code:

struct TwoInts
{
	int val[2];
	int &operator [] (unsigned int);
	TwoInts(int, int);
};

int &TwoInts::operator [] (unsigned int i) { return val[i]; }

TwoInts::TwoInts(int a, int b) { val[0] = a; val[1] = b; }

TwoInts operator + (const TwoInts &a, const TwoInts &b)
{
	return TwoInts(a[0]+b[0], a[1]+b[1]);
}

int main(int argc, char *argv[]) { return 0; }

When I compile it, the only line of the + operator gives the following error four times:
14 C:\test\main.cpp passing `const TwoInts' as `this' argument of `int& TwoInts::operator[](unsigned int)' discards qualifiers
This despite the fact that it seems to me I'm doing nothing wrong. Sure, a and b are const, but I'm not modifying them, nor am I treating the float returned by the [] operator as non const. Any way to fix this?

You're trying to call a non const member function on a const reference.
You could try this way

Code:

#include <iostream>
#include <string>

using namespace std;

struct TwoInts
{
	int val[2];
	
	int operator [] (unsigned int) const;
	int & operator [] (unsigned int);
	
	TwoInts(int, int);
};

int  TwoInts::operator [] (unsigned int i) const { return val[i]; }
int & TwoInts::operator [] (unsigned int i) { return val[i]; }

TwoInts::TwoInts(int a, int b) { val[0] = a; val[1] = b; }

TwoInts operator + (const TwoInts &a, const TwoInts &b)
{
	return TwoInts(a[0]+b[0], a[1]+b[1]);
}

int main(int argc, char *argv[]) { 
    return 0; 
}

Kurt

Ah, now it compiles without errors.
I take it it automatically uses the const function whenever it's called on a constant instance, and always uses the non-const function otherwise?

Yes. The rule of thumb is that subscript operators often come in pairs.