Question

I have two questions related to function objects and function pointers,

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Question : 1

When I read the different uses sort algorithm of STL, I see that the third parameter can be a function objects, below is an example

class State {  
  public:  
    //...
    int population() const;  
    float aveTempF() const;  
    //...  
};    
struct PopLess : public std::binary_function<State,State,bool> {  
    bool operator ()( const State &a, const State &b ) const  
        { return popLess( a, b ); }  
};  
sort( union, union+50, PopLess() );  

Question :

Now, How does the statement, sort(union, union+50,PopLess()) work? PopLess() must be resolved into something like PopLess tempObject.operator() which would be same as executing the operator () function on a temporary object. I see this as, passing the return value of overloaded operation i.e bool (as in my example) to sort algorithm.

So then, How does sort function resolve the third parameter in this case?

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Question : 2

Question

Do we derive any particular advantage of using function objects versus function pointer? If we use below function pointer will it derive any disavantage?

inline bool popLess( const State &a, const State &b )
    { return a.population() < b.population(); }  
std::sort( union, union+50, popLess ); // sort by population

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PS : Both the above references(including example) are from book “C++ Common Knowledge: Essential Intermediate Programming” by “Stephen C. Dewhurst”.
I was unable to decode the topic content, thus have posted for help.

Thanks in advance for your help.

Answer 1

PopLess() instantiates a temporary instance of the class PopLess to be passed to std::sort(). It’s effectively the same as if you were to say (note that in this example an extra copy is made):

PopLess pl = PopLess();
sort(union, union + 60, pl);

Then, std::sort() will call the operator() on that instance.

As for whether function objects or function pointers are better “better,” it depends. Probably the most important difference is that function objects can maintain state while ordinary functions passed by pointer cannot. A compiler might be able to optimize one or the other better, but in most usage scenarios that’s probably not important.