fucntion object to check the value of a map element

[heatkernel]

In a book (Sec 6.6.5 of Jossoutis' book on the Standard Library), I find this code:

   /*function object to check the value of a map element
    */
   template <class K, class V>
   class value_equals {
     private:
       V value;
     public:
       //constructor (initialize value to compare with)
       value_equals (const V& v)
        : value(v) {
       }
       //comparison
       bool operator() (pair<const K, V> elem) {
           return elem.second == value;
       }
   };

which is invoked in the following way:

      //search an element with value 3.0
       pos = find_if (coll.begin(),coll.end(),    // linear complexity
                      value_equals<float,float>(3.0));

Can you explain why you call it this way?

How is the method "operator" invoked when you call the constructor "value_equals"?

Also, I do not understand the syntax of the constructor...why is there an & after the V in the definition of the constuctor?

It also seems to have an empty body. Could someone explain that?

Thanks!

[Bazzy]

See http://www.cplusplus.com/reference/algorithm/find_if/

[heatkernel]

Thanks, Bazzy,

I should have made clear that I understand the basic use of "find_if", as in the example you provided, where the third argument is a Boolean function. The part I don't understand is how one interprets the class definition in the first block so that:


value_equals<float,float>(3.0)

is a Boolean function. The fact that the above is used inside a find_if command is really incidental to my question...I hope this clarifies my question.

Another way of putting it is that as far as I can see the ()'s around the 3.0 in the above line of code are part of the call to the class constructor value_equals. But there also must be ()'s that are interpreted as the overloaded ()'s defined in the method definition

bool operator() (pair<const K, V> elem) {
           return elem.second == value;
       }

I don't see where those overloaded parentheses arise in this invocation. Or are they the parentheses in the line: "value(v)"?

Thanks!

[Bazzy]

read more carefully what the reference page says ( and the example implementation )
You are passing a functor ( http://en.wikipedia.org/wiki/Function_object#In_C_and_C.2B.2B )

[heatkernel]

OK, thanks, understand it now.