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# Function taking as an argument std::equal_to, std::grater etc

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4 replies to this topic

### #1Misery  Members

Posted 18 July 2012 - 05:22 AM

Hello,

I am trying to make a function that would take some kind of comparision operator from std:: library. For example:

   std::equal_to

as one of the arguments.
However, I fail all the time. Could someone give an example of such function, that would work?

Regards

### #2mrbastard  Members

Posted 18 July 2012 - 05:25 AM

template <typename T>
bool doStuff(int a, int b, T comparitor)
{
return comparitor(a,b);
}


I'm guessing the problem you're having is that the component std functions / functors are not of the same type. Template parameters make this problem go away.

Some of the std component functions may be of related types : e.g. deriving from std::unary_function etc - so it may be possible to use normal polymorphism to work around this - I wouldn't recommend it though. TBH I'm not even sure inheriting from bases like std::unary_function is enforced by the standard.

Lastly, you could take an std::function<bool (int,int)> or similar, if you want to avoid writing templates.

Edited by mrbastard, 18 July 2012 - 05:32 AM.

### #3Matt-D  Members

Posted 18 July 2012 - 02:19 PM

A good place to look for examples (and inspiration) are the docs for SGI STL: http://www.sgi.com/tech/stl/
Often you can find something similar to what you're trying to accomplish. In your case std::count_if (for instance) takes a "Predicate pred" parameter, just like you want in case of std::equal_to being an argument: http://www.sgi.com/t...l/count_if.html

[source lang="cpp"]template &lt;class InputIterator, class Predicate, class Size&gt;void count_if(InputIterator first, InputIterator last, Predicate pred, Size& n);[/source]

Note that in the context of template parameters specification (the first line, the one that starts with "template") you can either use "class" (you are allowed to pass a built-in type, like "std::size_t" for "Size" or "int *" for "InputIterator", so it's a bit of a misnomer) or "typename" pretty much interchangeably (the only case where you have to use the "class" are the so-called TTPs (template-template parameters, like when you want to pass a class template "std::vector" as opposed to, say, template class "std::vector<double>"), but don't worry about it here).

Another reason I point out the docs is that they show good naming conventions for template parameters -- it's good idea to use names that express the concept, like "Predicate", instead of general names, like "T", for self-documenting code. Even though it's "just" for documentation (it won't be enforced, concepts as a type enforcement mechanism didn't get into C++11), it's pretty useful -- it allows the code reader (often you) to just quickly skim at the function signature to figure out what it does and what it wants (just like in the above "count_if" example). You can look up the concept you need by looking up the class (template) you want to use, for instance, http://www.sgi.com/t...l/equal_to.html

Another good example of using a concept name to document your requirements is std::sort, http://www.sgi.com/tech/stl/sort.html -- that's the relevant example for arguments like std::less:

[source lang="cpp"]template &lt;class RandomAccessIterator, class StrictWeakOrdering&gt;void sort(RandomAccessIterator first, RandomAccessIterator last, StrictWeakOrdering comp);[/source]

This makes clear to the users of std::sort that they are expected to pass a particular comparator function object -- one consistent with a strict weak ordering -- not just "any" comparator.

http://www.sgi.com/t...l/functors.html
http://www.sgi.com/t...yPredicate.html

BTW, note that unary_function & binary_function are deprecated in C++11:
http://www.open-std....2010/n3145.html
http://en.cppreferen.../unary_function

Edited by Matt-D, 18 July 2012 - 02:47 PM.

### #4mrbastard  Members

Posted 18 July 2012 - 02:43 PM

BTW, note that unary_function & binary_function are deprecated in C++11:
http://www.open-std....2010/n3145.html
http://en.cppreferen.../unary_function

Nice to know, thanks!

### #5Misery  Members

Posted 19 July 2012 - 01:21 AM

Thanks a lot :]
Your suggestions Gyus, did the job!

Edited by Misery, 19 July 2012 - 01:21 AM.

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