overloading > as member function

Question

I have written a relational operator < as member of class Test

bool Test::operator<(const Test& t) { if (a<t) return true; }

this code is in the header file, which I have included in my .cpp. However, when I compile my program, I get the following error:

test.h: 134:6: error: ‘a’ was not declared in this scope

Where do I declare 'a'? should I write it in my header file as Test& a? can you please help me fix this. thx!

What is a supposed to be? A data member of class Test? And why are you comparing it against a Test object? — Ray Toal
– Ray Toal, Commented Aug 20, 2011 at 23:00
The compiler is confused as to what you mean by a in a<t. So are we. Please explain. — hmakholm left over Monica
– hmakholm left over Monica, Commented Aug 20, 2011 at 23:01
The member operator only takes one argument, the right-hand side, and x < y invokes x.operator<(y)... so you need to compare your argument t to the instance itself. — Kerrek SB
– Kerrek SB, Commented Aug 20, 2011 at 23:01
@Kerrek That's correct but hopefully the OP will not run with this advice in the absolute literal sense, because an implementation like *self<t would be circular. :-) — Ray Toal
– Ray Toal, Commented Aug 20, 2011 at 23:09
@Ray: sure - since the OP is overloading a comparison operator, I'm sort of assuming that she knows that the purpose of this is to define the comparison semantics. By the way, in C++ it's called this (not "self") :-) — Kerrek SB
– Kerrek SB, Commented Aug 20, 2011 at 23:12

koan · Accepted Answer · 2011-08-21 09:08:00Z

You're supposed to be defining how a Test object may be compared to another object of type Test but in your code you do not define how, only that "a" - whatever that is, is less than the other object.

class Test { public: Test(int myscore) { score = myscore; } bool operator<(const Test &t); int score; } bool Test::operator<(const Test &t) { // Is less than if score is smaller if(score < t.score) return true; else return false; }

Then in your program,

// ... Test test1(4); Test test2(5); if(test1 < test2) std::cout << "4 is less than 5 by comparing objects\n"; else std::cout << "Failed!\n";

You should either return false; if the if is false or just use return score < t.score to begin with. Otherwise you'll get strange behavior.
Good catch @Voo, I was trying too hard to make the given code work. It should have been a compiler error when the function didn't return false.
If you make operator< a member of the class, you should probably make it const, too.

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