Sorting a vector of objects using a compare function

Question

I am trying to follow a link where a someone is trying to simplify the C++ sort reference http://www.cplusplus.com/forum/beginner/4817/ however I can not get the bool operator function to compile the way he has it.

I have vector of event objects. I want to sort the vector based on the event begin time. If the begin times were numbers, this would be easier but they are strings so I had to write functions to convert to uint64_t, all of my code up to this sort attempt is working as it should. Here is the code I am trying to get to work:

The bool function:

bool EWriter:: operator () ( Event &a,  Event &b){
    return (stringToTime(stringReturnWrap(a.getBeginTime())) < stringToTime(stringReturnWrap(b.getBeginTime()))); 
}

This code compiles but I can not figure out how to give it a name, and therefore I cannot reference it in the sort. Also, I would rather overload the < operator but I keep getting an error it needs a third argument.

Here is my sort:

sort(events->begin(), events->end(), someFunctionName);??

Slightly unrelated is that I know I am supposed to use const in the args but I can not call the functions of the Event class if I have them implemented.

Did you forget to add const-ness to your Event &a and Event &b? — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Dec 20, 2011 at 20:45
Event::getBeginTime() should be declared const. If it's not your code, maybe you could get the author to fix it. If it IS your code, FIX IT! 8v) — Fred Larson
– Fred Larson, Commented Dec 20, 2011 at 20:46

Drew Dormann · Accepted Answer · 2011-12-20 20:46:25Z

2

By overloading operator(), you have made EWriter a functor.

Just pass it an instance of EWriter.

If EWriter has a default constructor, you can use:

sort(events->begin(), events->end(), EWriter());

Or pass it an EWriter that already exists.

answered Dec 20, 2011 at 20:46

Drew Dormann

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1 Comment

Miek Over a year ago

I was trying to follow an example that was not correct. I should have overloaded the < operator, but the compiler said I needed a third argument to do that and I could not figure out what that would be.

rendon · Accepted Answer · 2011-12-20 20:48:50Z

0

You are overloading the parentheses operator:

bool EWriter:: operator () ( Event &a,  Event &b){
    return (stringToTime(stringReturnWrap(a.getBeginTime())) < stringToTime(stringReturnWrap(b.getBeginTime())));

}

Try with:

bool EWriter:: operator <( Event &a,  Event &b){
    return (stringToTime(stringReturnWrap(a.getBeginTime())) < stringToTime(stringReturnWrap(b.getBeginTime())));

}

answered Dec 20, 2011 at 20:48

rendon

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2 Comments

Miek Over a year ago

I tried that, the compiler kept telling me I needed a third arg

Miek Over a year ago

The answer that got deleted said to make a function basically like what you have only name it compare and remove the operator overload all together. then the vector sort is std::sort(eventVentor.begin(), eventVector.end, compare). That made sense but I got frustrated and wrote a bubble sort that seems to be working. Thanks for the help

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