Cannot call front() on queue in C++ - c++

Question

I am attemtping to access and pop the front element from a queue in C++, but the line
tuple<int, string, future<string>> post = posterVerificationQueue.front();
is underlined in red and gives the error:
function "std::tuple<_This, _Rest...>::tuple(const std::tuple<_This,
_Rest...> &) [with _This=int, _Rest=<std::string, std::futurestd::string>]" (declared at line 320 of "C:\Program Files
(x86)\Microsoft Visual
Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\tuple") cannot
be referenced -- it is a deleted function
I imagine it is an issue with me using a future within a tuple, but not too sure. Here is my code:
#include <iostream>
#include <ctime>
#include <chrono>
#include <thread>
#include <future>
#include <queue>
#include <mutex>
#include <tuple>
#include <map>
#include "TCPClient.h"
#include "ThreadPool.h"
#include "RequestGenerator.h"
#include "ResponseVerifier.h"
#include "Storage.h"
#define DEFAULT_PORT 12345
void readRequest(string serverIp, int threadIndex, double timeDurationSecs);
void postRequest(string serverIp, int threadIndex, double timeDurationSecs);
int readRequests = 0;
int postRequests = 0;
mutex mLock;
map<int, tuple<double, int>> posterThreadMap;
queue<tuple<int, string, future<string>>> posterVerificationQueue;
map<int, tuple<double, int>> readerThreadMap;
Storage* db = new Storage();
RequestGenerator* requestGenerator = new RequestGenerator();
int main(int argc, char **argv)
{
// Default parameters
unsigned int posterCount = 5;
unsigned int readerCount = 0;
double timeDurationSecs = 10;
bool throttle = false;
string serverIp = "127.0.0.1";
// Validate the parameters
if (argc != 6) {
std::cout << "\nUsage (required parameters): server_IP number_of_poster_threads number_of_reader_threads time_duration throttle(0|1)\n\n";
std::cout << "server_IP - IP of the server\n";
std::cout << "number_of_poster_threads - number of threads performing POST operations\n";
std::cout << "number_of_reader_threads - number of threads performing READ operations\n";
std::cout << "time_duration - duration of test execution in seconds\n";
std::cout << "throttle(0|1) - 0: do not throttle message speed\n";
std::cout << "\t\t1: throttle message speed\n\n";
std::cout << "\nDefault Parameters:\n";
std::cout << "\tserver_IP - " << serverIp << "\n";
std::cout << "\tnumber_of_poster_threads - " << posterCount << "\n";
std::cout << "\tnumber_of_reader_threads - " << readerCount << "\n";
std::cout << "\ttime_duration - " << timeDurationSecs << "s\n";
std::cout << "\tthrottle - " << (throttle ? "true" : "false") << "\n\n";
std::cout << "Enter dev mode using default paramaters?\n";
system("pause");
}
else
{
serverIp = argv[1];
posterCount = (int)argv[2];
readerCount = (int)argv[3];
timeDurationSecs = (int)argv[4];
throttle = (int)argv[5];
}
cout << "\nStarting throughput test...\n";
ThreadPool posterPool(posterCount);
vector<future<void>> posterFutures;
vector<tuple<string, int, int>> incorrectPostResponses;
double posterTotalTime = 0.0; // The total time in seconds that all poster threads took to run
ThreadPool readerPool(readerCount);
vector<future<void>> readerFutures;
double readerTotalTime = 0.0; // The total time in seconds that all reader threads took to run
for (int i = 0; i < posterCount; i++)
posterFutures.push_back(posterPool.enqueue(postRequest, serverIp, i, timeDurationSecs));
for (int i = 0; i < readerCount; i++)
readerFutures.push_back(readerPool.enqueue(readRequest, serverIp, i, timeDurationSecs));
for (int i = 0; i < posterFutures.size(); i++)
posterFutures[i].wait();
for (int i = 0; i < readerFutures.size(); i++)
readerFutures[i].wait();
for (int i = 0; i < posterThreadMap.size(); i++)
{
double posterRequestsPerSecond = get<1>(posterThreadMap[i]);
double threadRunTime = get<0>(posterThreadMap[i]);
posterTotalTime += threadRunTime;
std::cout << "\nPoster thread " << i << " (ran for " << threadRunTime << "s) - Average post requests per second: " << posterRequestsPerSecond << "\n";
}
// Should verification be optional?
while (!posterVerificationQueue.empty())
{
tuple<int, string, future<string>> post = posterVerificationQueue.front();
posterVerificationQueue.pop();
int postIndex = get<0>(post);
string request = get<1>(post);
string response = get<2>(post).get();
tuple<bool, int, int> postVerification = db->addPosterValue(postIndex, request, response);
bool isValid = get<0>(postVerification);
if (!isValid)
{
int correctResponse = get<1>(postVerification);
int actualResponse = get<2>(postVerification);
incorrectPostResponses.push_back(make_tuple(request, correctResponse, actualResponse));
}
}
cout << "\nTotal poster runtime: " << posterTotalTime << "s" << "\n";
cout << "\nTotal post requests: " << postRequests << "\n";
cout << "\nAverage post requests per second per thread: " << postRequests / posterTotalTime << "\n";
cout << "\nIncorrect responses: " << incorrectPostResponses.size() << "\n";
for (int i = 0; i < incorrectPostResponses.size(); i++)
{
tuple<string, int, int> incorrectResponse = incorrectPostResponses[i];
string request = get<0>(incorrectResponse);
int correctResponse = get<1>(incorrectResponse);
int actualResponse = get<2>(incorrectResponse);
cout << "Incorrect response #" << i + 1 << "\n";
cout << "Request: " << request << "\n";
cout << "Expected response: " << correctResponse << "\n";
cout << "Actual response: " << actualResponse << "\n\n";
}
// TODO: Implement the block above for reader threads
delete db;
delete requestGenerator;
system("pause");
return 0;
}
string sendRequest(TCPClient client, string request)
{
return client.send(request);
}
void postRequest(string serverIp, int threadIndex, double timeDurationSecs)
{
TCPClient client(serverIp, DEFAULT_PORT);
client.OpenConnection();
int threadPostCount = 0;
double timeSpan;
chrono::high_resolution_clock::time_point endTime;
chrono::high_resolution_clock::time_point startTime = chrono::high_resolution_clock::now();
do
{
/*
Could limit with
if (!throttle || (throttle && threadPostCount < (timeDurationSecs * 1000)))
{}
*/
string request = requestGenerator->generateWriteRequest();
mLock.lock();
future<string> responseFut = async(launch::async, sendRequest, client, request);
posterVerificationQueue.push(make_tuple(postRequests, request, move(responseFut)));
postRequests++;
mLock.unlock();
threadPostCount++;
endTime = chrono::high_resolution_clock::now();
timeSpan = chrono::duration_cast<chrono::duration<double>>(endTime - startTime).count();
} while (timeSpan < timeDurationSecs);
double totalRunTime = (endTime - startTime).count();
double posterRequestsPerSecond = threadPostCount / timeSpan;
tuple<double, double> returnValues = make_tuple(timeSpan, posterRequestsPerSecond);
mLock.lock();
postRequests += threadPostCount;
posterThreadMap[threadIndex] = returnValues;
mLock.unlock();
client.CloseConnection();
}
void readRequest(string serverIp, int threadIndex, double timeDurationSecs)
{
//return 0.0;
}

Answer 1

std::tuple<_This, _Rest...>::tuple(const std::tuple<_This, _Rest...> &)
...
cannot be referenced -- it is a deleted function
Well, that's the tuple copy constructor, right?
I imagine it is an issue with me using a future within a tuple, but not too sure.
Let's check the documentation for the constructors of std::future ...
future( const future& other ) = delete;
std::future is not CopyConstructible.
So I'd say your imagination was correct. You can't copy the tuple, because it can't copy one of its members.
You can use std::move to move construct from the queue element, but remember it would be a problem if you didn't pop the front immediately after (in the code shown, you do this, so it's fine).