Can't sort list with custom comparator function

Question

I'm getting this compiler error when I try to sort my ingredients with my custom comparator.

kitchen.cpp: In member function ‘void Kitchen::printContents(std::ofstream&)’:
kitchen.cpp:172: error: no matching function for call to ‘std::list<Ingredient, std::allocator<Ingredient> >::sort(<unresolved overloaded function type>)’
/usr/include/c++/4.2.1/bits/list.tcc:271: note: candidates are: void std::list<_Tp, _Alloc>::sort() [with _Tp = Ingredient, _Alloc = std::allocator<Ingredient>]
/usr/include/c++/4.2.1/bits/list.tcc:348: note:                 void std::list<_Tp, _Alloc>::sort(_StrictWeakOrdering) [with _StrictWeakOrdering = bool (Kitchen::*)(const Ingredient&, const Ingredient&), _Tp = Ingredient, _Alloc = std::allocator<Ingredient>]

Here's the code that is causing it:

bool sortFunction(const Ingredient a, const Ingredient b)
{
    if (a.getQuantity() < b.getQuantity())
        return true;
    else if (a.getQuantity() == b.getQuantity())
    {
        if (a.getName() < b.getName()) return true;
        else return false;
    }
    else return false;
}

void Kitchen::printContents(std::ofstream &ostr)
{
    ostr << "In the kitchen: " << std::endl;

    ingredients.sort(sortFunction);

    std::list<Ingredient>::iterator itr;
    for (itr = ingredients.begin(); itr != ingredients.end(); ++itr)
    {
        ostr << std::setw(3) << std::right << itr->getQuantity() << " " 
        << itr->getName() << std::endl;

    }
}

how are you defining sort? The error is saying that you don't have a matching call to it. — CoderDake
– CoderDake, Commented Feb 28, 2013 at 18:21
@TopGunCoder: std::list has a sort() function that takes a comparator. — Fred Larson
– Fred Larson, Commented Feb 28, 2013 at 18:25
Do you have any other function named sortFunction? Is sortFunction a non-static member of Kitchen? — n. m. could be an AI
– n. m. could be an AI, Commented Feb 28, 2013 at 18:31
Why do you have both bool sortFunction(const Ingredient a, const Ingredient b) and also bool Kitchen::sortFunction(const Ingredient&, const Ingredient&)? — Mooing Duck
– Mooing Duck, Commented Feb 28, 2013 at 18:54

Community · Accepted Answer · 2017-05-23 11:43:21Z

3

There may be another sortFunction somewhere (e.g. in Kitchen), which causes the above error.

Try

ingredients.sort(::sortFunction);

2 Comments

Slava Over a year ago

Are you sure that the reason?

Logan Shire Over a year ago

I had it like that before and it was giving me the same error. : (

Slava · Accepted Answer · 2013-02-28 18:36:41Z

2

Looks like you have method in Kicthen called sortFunction, and compiler cannot choose proper one. You can try this:

list.sort( ::sortFunction );

To resolve it, or if the function you provided suppose to be method of Kitchen class you need to fix that.

Btw:

if (a.getName() < b.getName()) return true;
else return false;

Is the same as:

return a.getName() < b.getName();

edited Feb 28, 2013 at 18:36

answered Feb 28, 2013 at 18:27

Slava

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Comments

Mike Seymour · Accepted Answer · 2013-02-28 18:31:47Z

1

My guess is that you declare a member function Kitchen::sortFunction. Within another member function (such as printContents), that will hide the non-member function you want to use.

The error message suggests that this is the case; it is trying to instantiate sort for a member-function type bool (Kitchen::*)(const Ingredient&, const Ingredient&).

If the member function is not supposed to exist, then remove the declaration. If it is, then either rename one of the functions, or refer to the non-member function as ::sortFunction.

answered Feb 28, 2013 at 18:31

Mike Seymour

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Comments

Jamin Grey · Accepted Answer · 2013-02-28 18:42:09Z

0

Your sort function is:

bool sortFunction(const Ingredient a, const Ingredient b)

But it should probably be:

bool sortFunction(const Ingredient &a, const Ingredient &b)

(Note the references)

Also, as already mentioned, your Kitchen class already has a function called sortFunction() and it's taking precedence, so either use ::sortFunction() or give each function a unique and more-descriptive name.

If Kitchen::sortFunction() is the one you're wanting, it'll need to be a static member function.

answered Feb 28, 2013 at 18:42

Jamin Grey

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