Prefer function-pointer or function-object when customizing sort and map?

Question

I want to customizing sort template and map template in C++

Here are for comparing,

struct Greater1
{
    bool operator() (string A, string B)
    {
        string AB = A + B;
        string BA = B + A;
        return (AB >BA);
    }
};

static bool Greater2(string A, string B)
{
    string AB = A + B;
    string BA = B + A;
    return (AB >BA);
}

After my test Greater1 works for map and Greater2 works for sort. I also got some information form CPLUSPLUS and found that both and map should use both of function pointer and function object. My question is why Greater2 could work for map and Greater1 could work for sort.

Depending on your use, both should work equally well for any purpose. Please create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve) and show us how you use the structure and function. — Some programmer dude, Feb 17 '15 at 00:43
@JoachimPileborg: If I recall, functionoid-objects can have performance/size differences, that _may_ have an effect. — Mooing Duck, Feb 17 '15 at 00:56
@JoachimPileborg I could not even compile following code successfully, sort(str.begin(), str.end(), Greater1); or map minmap; — Yutao Xing, Feb 17 '15 at 00:57
You may prefer a function object instead of a function pointer because the former can be inlined. This may make a difference, and it does e.g. in `std::sort` vs the C-style version `qsort` — vsoftco, Feb 17 '15 at 00:58
Then edit your question to include the code that causes the error(s) ***and*** the actual errors (complete and unmodified). — Some programmer dude, Feb 17 '15 at 00:58

Pradhan · Answer 1 · 2015-02-17T01:04:24.110

3

std::sort takes an instance of a Compare object, while the class template std::map allows you to specify the type of the Compare. You likely tried to use Greater1, a type, like this:

sort(beginIt, endIt, Greater1);

which is incorrect. You should use it as

sort(beginIt, endIt, Greater1());

However, since Greater2 is a function pointer and not a type name,

sort(beginIt, endIt, Greater2);

works fine.

With an std::map, you need to provide a type. So, you would need to use something like Greater1. You could also use the std::map constructor that allows you to specify the Compare object and some template magic to end up using Greater2 like this:

template <typename Key, typename Value, typename Compare>
std::map<Key, Value, Compare> make_map(Compare const& comp)
{
 return std::map<Key, Value, Compare>(comp);
}

And you can now make a map like this:

auto myMap = make_map<std::string, std::string>(Greater2);

edited Feb 17 '15 at 01:04

answered Feb 17 '15 at 00:55

Pradhan

16,391
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The type *may* not be needed for the `std::map`, if the comparator function is passed to the [`std::map` constructor](http://en.cppreference.com/w/cpp/container/map/map). – Some programmer dude Feb 17 '15 at 00:57
2

@JoachimPileborg: Which is tricky to do without writing a type: `std::map(Greater1{});` Not impossible definitely, but tricky. – Mooing Duck Feb 17 '15 at 00:58
@MooingDuck Note that I say *may* :) For the function, if the arguments match standard type used for the map, then you don't need the type – Some programmer dude Feb 17 '15 at 00:59
@Pradhan You can use `std::function` as I've in my answer – P0W Feb 17 '15 at 01:05
1

@P0W Ah right. That is probably better than my convoluted `make_map` :) – Pradhan Feb 17 '15 at 01:11

P0W · Accepted Answer · 2015-02-17T01:10:51.207

2

For interchanging function object and pointer with std::map you need to specify type for function pointer, like following

typedef std::function< bool(string,string)> compare;

Then,

std::map<string, string, compare >  m(Greater2) ;

For std::sort

sort(beginIt, endIt, Greater1() ); // since Greater1 is type

or

sort(beginIt, endIt, Greater2 );

edited Feb 17 '15 at 01:10

answered Feb 17 '15 at 01:04

P0W

46,614
9
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Thank you for your reply. Just one more question, why map needs to know the type of function pointer, whereas sort does not? why sort needs to know type of function object(why needs "()"), whereas map does not? Thank you so very much!! – Yutao Xing Feb 17 '15 at 02:44

Prefer function-pointer or function-object when customizing sort and map?

2 Answers2