Linkediting error when having a static unique_ptr [duplicate] - c++

Question

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Undefined reference to static class member
(9 answers)
Undefined reference to a static member
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Closed last year.
Because I want my unique_ptr to be the same in all my objects, I decided to make it a static field.
After that an error has appeared:
https://pastebin.com/PPe6z0Ub
I don't really know how to fix it, I've tried searching for it online but I couldn't find anything really useful.
Code:
DBFeed.h
#pragma once
#include <vector>
#include <string>
#include <memory>
class DBFeed
{
public:
void CalculatePosts();
std::vector<std::string> GetPost();
void ClearFeed();
private:
static std::unique_ptr<std::vector<std::vector<std::string>>> m_postari;
static int m_index;
};
DBFeed.cpp
#include "DBFeed.h"
#include "DBLink.h"
int DBFeed::m_index = 0;
void DBFeed::CalculatePosts()
{
DBFeed::m_postari = std::make_unique<std::vector<std::vector<std::string>>>();
pqxx::work worker(*DBLink::GetInstance());
try
{
worker.conn().prepare("CountPostsFeed", "SELECT COUNT(id) FROM posts");
worker.conn().prepare("ReadAllPostsFeed",
"SELECT message FROM followers RIGHT JOIN posts ON followers.followed = posts.user_id WHERE followers.follower = 51 LIMIT 5 OFFSET" + std::to_string(m_index+=5)
);
pqxx::row countPostsResult = worker.exec_prepared1("CountPostsFeed");
std::string number = countPostsResult.at("count").c_str();
pqxx::result readAllPostsResult = worker.exec_prepared_n(std::stoi(number), "ReadAllPostsFeed");
for (const auto& currentPost : readAllPostsResult)
{
std::string postID = currentPost.at("id").c_str();
std::string message = currentPost.at("message").c_str();
std::string dateCreated = currentPost.at("date_created").c_str();
DBFeed::m_postari->push_back(std::vector<std::string>{postID, message, dateCreated});
}
worker.commit();
}
catch (const pqxx::unexpected_rows&)
{
throw std::runtime_error("Reading all of an users posts for feed failed");
}
}
std::vector<std::string> DBFeed::GetPost()
{
if (m_postari->empty())
{
CalculatePosts();
}
std::vector<std::string> deReturnat = m_postari->at(0);
m_postari->erase(m_postari->begin());
return deReturnat;
}
void DBFeed::ClearFeed()
{
m_index = 0;
m_postari.reset();
}
Why is this error happening and how can I fix it?
//Edit:
Fixed it by adding this by changing DBFeed.cpp's first lines as follows:
#include "DBFeed.h"
#include "DBLink.h"
int DBFeed::m_index = 0;
std::unique_ptr<std::vector<std::vector<std::string>>>
DBFeed::m_postari = std::make_unique<std::vector<std::vector<std::string>>>();
void DBFeed::CalculatePosts()
{
pqxx::work worker(*DBLink::GetInstance());
....
But I'd still like to get an explanation on why it was happening and why is it fixed now.