“no match for 'operator<'” when trying to insert to a std::set

I'm using gcc 4.3.3 to try to compile the following code:

struct testStruct {  
int x;  
int y;  
bool operator<(testStruct &other) { return x < other.x; }  
testStruct(int x_, int y_) {  
    x = x_;  
    y = y_;  
}  
};  


int main() {
multiset<testStruct> setti;  
setti.开发者_运维百科insert(testStruct(10,10));  
return 0;  
}

I get this error:

/usr/include/c++/4.4/bits/stl_function.h|230|error: no match for ‘operator<’ in ‘__x < __y’

I suspect I'm not doing the the operator overloading as it should be done, but I just can't pinpoint the exact problem. What am I doing wrong here?

The operator must be const and take a const reference:

bool operator<(const testStruct &other) const { return x < other.x; }