How to use Function Pointer as a parameter in a Class - c++

Question

I am trying to construct a class that is essentially a queue of futures that are all handled asynchronously and eventually all stored when main wants to get their values. I am having trouble creating the function parameters that will accept these functions and their arguments in order to create an async operation that is then pushed in to the queue of futures. The areas in question are the call constructor and the member function add(). Here is what I have so far:
#include <iostream>
#include <queue>
#include <future>
#include <thread>
using namespace std;
using longInt = unsigned long long int;
//prototypes
longInt isPrime(longInt);
template <typename return_type>
class TaskQueue {
private:
queue<future<return_type>> tasks;
public:
//return a copy of the queue
queue<future<return_type>> copy() const {
return tasks;
}
//default constructor
//does nothing, waits for input
TaskQueue() {
//do nothing
}
//call constructors
//adds task to queue
TaskQueue(return_type (*func)(), Args&& ... args) {
tasks.push(new async(func, args));
}
//copy constructor
//copies another queue to this one
TaskQueue(const queue<future<return_type>> & in) {
tasks = in.copy();
}
//setter and getter functions
//inserts a new task into the queue
void add(return_type(*func)(), Args&& ... args) {
tasks.push(new aync(in, args));
}
//returns true if the task at the top of the queue is ready
bool valid() {
return tasks.front().valid();
}
//gets the value, if the value is not ready, waits for it to be ready
//pops the top task after getting it
return_type get() {
return_type temp = tasks.top().get();
tasks.pop();
return temp;
}
//waits for the value of the top of the queue to become ready
void wait() {
tasks.top().wait();
}
};
int main() {
TaskQueue<longInt> checkPrimes;
checkPrimes.add(isPrime, 5);
longInt test = checkPrimes.get();
cout << test << endl;
}
//returns the number if it is prime or 0 if it is not
longInt isPrime(longInt n) {
if (n <= 3) {
if (n > 1) {
return n;
}
return 0;
}
if (n % 2 == 0 || n % 3 == 0) {
return 0;
}
for (unsigned short i = 5; i * i <= n; i += 6) {
if (n % i == 0 || n % (i + 2) == 0) {
return 0;
}
}
return n;
}

Answer 1

A compilable version:
template <typename return_type>
class TaskQueue {
private:
queue<future<return_type>> tasks;
public:
//return a copy of the queue
queue<future<return_type>> copy() const {
return tasks;
}
//default constructor
//does nothing, waits for input
TaskQueue() {
//do nothing
}
//call constructors
//adds task to queue
template <typename ... Args, typename ... Ts>
TaskQueue(return_type (*func)(Ts...), Args&& ... args) {
tasks.push(std::async(func, args...));
}
//copy constructor
//copies another queue to this one
TaskQueue(const queue<future<return_type>> & in) {
tasks = in.copy();
}
//setter and getter functions
//inserts a new task into the queue
template <typename ... Args, typename ... Ts>
void add(return_type(*func)(Ts...), Args&& ... args) {
tasks.push(std::async(func, args...));
}
//returns true if the task at the top of the queue is ready
bool valid() {
return tasks.front().valid();
}
//gets the value, if the value is not ready, waits for it to be ready
//pops the top task after getting it
return_type get() {
return_type temp = tasks.front().get();
tasks.pop();
return temp;
}
//waits for the value of the top of the queue to become ready
void wait() {
tasks.top().wait();
}
};
Demo

Answer 2

I think you can define a type for the function you want to pass and then apply them in the order you want. For example, the following snippet will define a intfunc type, which is a function that takes an int and returns an int.
typedef int (*intfunc) (int);
We can then define functions like
int sum2(int n) {
return n + 2;
}
int mult3(int n) {
return 3 * n;
}
and give them to a vector,
vector<intfunc> funcs;
vector<intfunc>::iterator func;
int start = 0;
funcs.push_back(sum2);
funcs.push_back(mult3);
cin >> start;
for (func = funcs.begin(); func != funcs.end(); ++func)
{
start = (*func)(start);
}
cout << start << endl;
If we enter 5 to that program, it will return 21 as expected.