Overloading gives me error: no match for ‘operator<’

Question

In code I have queue of Events sorted by waitTime. I want to find which Events should be executed for the current moment, so I do this:

std::vector<Event>::iterator up = std::upper_bound(queue.begin(), queue.end(), currentTime);

The std::upper_bound will work if I overload < operator:

bool Event::operator<(const double& currentTime) const
{
    return waitTime < currentTime;
}

But I have an error:

   error: no match for ‘operator<’ (operand types are ‘const double’ and ‘Event’)

How should I correctly overload ‘operator<’ ?

P.S

class Event{
public:
    double startTime;
    double waitTime;
    double size;

    Event(double start, double wait, double size);
    bool operator<(const Event& otherEvent) const;
    bool operator<(const double& currentTime) const;
    bool operator() (const Event & event, const double & right);
};

btw looks extremely fishy that you use `<=` to implement `<`, in general those two impose quite different orderings — 463035818_is_not_an_ai, Dec 01 '17 at 13:40
Note that once you get this to compile, there's still a serious problem: `waitTime <= currentTime` is not a strict weak ordering, so the behavior of the code is undefined. The problem is that when `waitTime` is equal to `currentTime`, that `operator<` reports **both** that `waitTime` comes before `currentTime` **and** that `currentTime` comes before `waitTime`. — Pete Becker, Dec 01 '17 at 13:40

Vlad from Moscow · Accepted Answer · 2017-12-01T13:52:49.943

Taking into account this error message

error: no match for ‘operator<’ (operand types are ‘const double’ and ‘Event’)

you need to declare the operator

bool operator<(const double &, const Event &);

It seems that within the algorithm there is used the condition

currentTime < *it

Another approach is to call the algorithm like

std::vector<Event>::iterator up = std::upper_bound(queue.begin(), 
                                                   queue.end(), 
                                                   Event { 0.0, currentTime, 0.0 });

that is by using casting of currentTime to an object of the type Event because there is already overloaded operator < for objects of the type Event..

bool operator<(const Event& otherEvent) const;

score 2 · Answer 2 · answered Dec 01 '17 at 13:46

This might be really useful. The following link deals with global operator overrides, and the limitations thereof, as well as the use of the friend keyword in C++ 11.

[link] Why should I overload a C++ operator as a global function (STL does) and what are the caveats?

AndyG · Answer 3 · 2017-12-01T13:57:08.340

bool Event::operator<(const double& currentTime) const

Defines the less-than operator for only the following situation:

Event e;
//...
double d = /*...*/;
bool result = e < d;

NOT the following situation

bool result = d < e;

When defining these operators, you MUST define them both ways! Ideally define them both as non-member functions

bool operator<(const Event& e, const double& currentTime);
bool operator<(const double& currentTime, const Event& e);

(Why nonmember functions? To improve encapsulation)

John Lakos has a has a wonderful CPPcon talk where he says exactly this.

Overloading gives me error: no match for ‘operator<’

3 Answers3