'future' has been explicitly marked deleted here - c++

Question

I am trying to build a Async application to allow processing of large lists in parallel, and after two days of learning C++ through googling I have come up with the title error, from the following code:
//
// main.cpp
// ThreadedLearning
//
// Created by Andy Kirk on 19/01/2016.
// Copyright © 2016 Andy Kirk. All rights reserved.
//
#include <iostream>
#include <thread>
#include <vector>
#include <chrono>
#include <future>
typedef struct {
long mailing_id;
char emailAddress[100];
} emailStruct ;
typedef struct {
long mailing_id = 0;
int result = 0;
} returnValues;
returnValues work(emailStruct eMail) {
returnValues result;
std::this_thread::sleep_for(std::chrono::seconds(2));
result.mailing_id = eMail.mailing_id;
return result;
}
int main(int argc, const char * argv[]) {
std::vector<emailStruct> Emails;
emailStruct eMail;
// Create a Dummy Structure Vector
for (int i = 0 ; i < 100 ; ++i) {
std::snprintf(eMail.emailAddress,sizeof(eMail.emailAddress),"user-%d#email_domain.tld",i);
eMail.mailing_id = i;
Emails.push_back(eMail);
}
std::vector<std::future<returnValues>> workers;
int worker_count = 0;
int max_workers = 11;
for ( ; worker_count < Emails.size(); worker_count += max_workers ){
workers.clear();
for (int inner_count = 0 ; inner_count < max_workers ; ++inner_count) {
int entry = worker_count + inner_count;
if(entry < Emails.size()) {
emailStruct workItem = Emails[entry];
auto fut = std::async(&work, workItem);
workers.push_back(fut);
}
}
std::for_each(workers.begin(), workers.end(), [](std::future<returnValues> & res) {
res.get();
});
}
return 0;
}
Really not sure what I am doing wrong, and have found limited answers searching. Its on OSX 10 if that is relevant, and XCode 7.

Answer 1

The future class has its copy constructor deleted, because you really don't want to have multiple copies of it.
To add it to the vector, you have to move it instead of copying it:
workers.push_back(std::move(fut));

Answer 2

This error can also be raised if you are passing a future object (within a thread) to a function which expects a pass by value.
For example, this would raise an error when you pass the future:
void multiplyForever(int x, int y, std::future<void> exit_future);
multiplyForever(3, 5, fut);
You can fix it by passing the future by reference:
void multiplyForever(int x, int y, std::future<void>& exit_future);
multiplyForever(3, 5, fut);