Using an STL algorithm on vector of pointers of an object (C++)

Question

I need to count the number of times an object of an array of pointers has the same name(member variable) as the parameter given to a member function. I have tried different approaches, but none of them worked. My code does not even compile. The error is: "error C2514: 'MyComparator' : class has no constructors" and here is the code of the class I use for the comparision and the function used for counting concurencies.

 //definition of the vector
 vector<Class1*> files;
 ...
 int Class2::countNames(string name)
    {
       return count_if(files.begin(), files.end(), bind2nd(MyComparator(),name));
    }
 ...
class MyComparator{
public:
 bool operator()(const CFileType*& ob1, const string &str)
 {
   return ob1->getName()==str;
 }
};

I am struggling with this for hours and I want to do it using STL. The catch here is that i have an vector of pointers, if I had a normal vector it would not need a predicate function, in my case I have to give a parameter to it and I think bind2nd() is the right way. Any help would be greatly appreciated!

Answer 1

Under C++11, you could use a lambda expression, which is much easier than the archaic bind2nd. For example:

#include <string>
#include <vector>
#include <algorithm>
#include <iostream>

class Class1 {
public:
    Class1(const std::string& name) : Name(name) {
    }
    const std::string& getName() const {
        return Name;
    }
private:
    std::string Name;
};

size_t CountMatchingNames(const std::vector<Class1*>& v, const std::string& name) {
    return std::count_if(
        v.begin(),
        v.end(),
        [&name](Class1* c1) { return name == c1->getName(); }
    );
}

int main() {

    Class1 rob("rob");
    Class1 bob("bob");
    Class1 mitch("mitch");

    std::vector<Class1*> v;
    v.push_back(&bob);
    v.push_back(&mitch);
    v.push_back(&rob);
    v.push_back(&mitch);
    v.push_back(&bob);
    v.push_back(&bob);

    std::cout << "rob count:\t" << CountMatchingNames(v, "rob") << std::endl;
    std::cout << "bob count:\t" << CountMatchingNames(v, "bob") << std::endl;
    std::cout << "mitch count:\t" << CountMatchingNames(v, "mitch") << std::endl;

    return EXIT_SUCCESS;

}

This prints:

rob count:      1
bob count:      3
mitch count:    2