Storing any functional objects in a variable

Consider the situation when you are developing a library that has its API declared in the header files and implementation in the source files. This library shall have a function that accepts any functional objects. Take a look at the following code:

// making a typedef for function pointer accepting int 
// and returning nothing 
typedef void (*func_t)(int); 
 
// Function that accepts pointer to function and 
// calls accepted function for each integer that it has.
// It cannot work with functional objects :( 
void process_integers(func_t f); 
 
// Functional object 
class int_processor { 
   const int min_; 
   const int max_; 
   bool& triggered_; 
 
public: 
    int_processor(int min, int max, bool& triggered) 
        : min_(min) 
        , max_(max) 
        , triggered_(triggered) 
    {} 
 
    void operator()(int i) const { 
        if (i...