Debugging Function Objects, Lambdas, and Callbacks

The following code contains a total of 3 errors. Find and fix them, then post in the community explaining what you found and how you fixed it.

Processor.h

#ifndef PROCESSOR_H
#define PROCESSOR_H
#include <iostream>
#include <functional>

class IntProcessor {
    int value;
public:
    IntProcessor(int v = 0)
        : value(v) {
    }
    struct Multiplier {
        int factor;

        Multiplier(int f = 1)
            : factor(f) {
        }

        int operator()(int x) const {
            return x * factor;
        }
    };

    int applyFunctor(const Multiplier& fun) const {
        return value;
    }

    int applyCallback(int (*callback)(int)) const {
        return callback;
    }

    int applyLambda(const std::function<int(int)>& func) const {
        return func();
    }
};
#endif