C++ Classes with operator() (Functors)


Example

Every class which overloads the operator() can be used as a function object. These classes can be written by hand (often referred to as functors) or automatically generated by the compiler by writing Lambdas from C++11 on.

struct Person {
    std::string name;
    unsigned int age;
};

// Functor which find a person by name
struct FindPersonByName {
    FindPersonByName(const std::string &name) : _name(name) {}

    // Overloaded method which will get called
    bool operator()(const Person &person) const {
         return person.name == _name;
    }
private:
    std::string _name;
};

std::vector<Person> v; // Assume this contains data
std::vector<Person>::iterator iFind =
    std::find_if(v.begin(), v.end(), FindPersonByName("Foobar"));
// ...

As functors have their own identity, they cannot be put in a typedef and these have to be accepted via template argument. The definition of std::find_if can look like:

template<typename Iterator, typename Predicate>
Iterator find_if(Iterator begin, Iterator end, Predicate &predicate) {
     for (Iterator i = begin, i != end, ++i)
         if (predicate(*i))
             return i;
     return end;
}

From C++17 on, the calling of the predicate can be done with invoke: std::invoke(predicate, *i).