Заголовочный файл stock.h
#ifndef STOCK_H
#define STOCK_H
#include <fstream>
#include <iostream>
#include "publishinghouse.h"

namespace stock
{
    extern std::vector<Book> booksStock;
    extern std::vector<PublishingHouse> phVector;

    void fillBooksStock();
    void fillPublishingHouse();
    void changeBooksStock();
}


Исходный код пространства имен stock stock.cpp
#include "stock.h"

std::vector<PublishingHouse> stock::phVector = {
    PublishingHouse("AST", 3),
    PublishingHouse("Exmo", 2),
    PublishingHouse("Onyx", 4),
    PublishingHouse("Machaon", 2),
    PublishingHouse("Egmont", 3)
};
std::vector<Book> stock::booksStock = {};

void stock::fillBooksStock()
{
    std::string line;
    std::string value;
    std::ifstream file("booksStock.txt");
    if (file)
    {
        while (std::getline(file, line))
        {
            if (line != "")
            {
                Book* book = new Book(std::atoi(line.c_str()));
                std::getline(file, line);
                book->set_Title(line);
                std::getline(file, line);
                book->set_Author(line);
                std::getline(file, line);
                book->set_Count(std::atoi(line.c_str()));
                std::getline(file, line);
                book->set_PicPath(line);
                std::getline(file, line);
                book->set_Genre(line);
                std::getline(file, line);
                book->set_PublishingHouse(line);
                std::getline(file, line);
                book->set_Rating(std::atoi(line.c_str()));
                std::getline(file, line);
                book->set_ReaderCategory(line);
                std::getline(file, line);
                book->set_CountOfPages(std::atoi(line.c_str()));
                std::getline(file, line);
                book->set_RetailMarkup(std::atoi(line.c_str()));
                std::getline(file, line);
                book->set_Price(std::atoi(line.c_str()));
                std::getline(file, line);
                if (line == "True")
                    book->set_IsBookNew(true);
                else
                    book->set_IsBookNew(false);
                booksStock.push_back(*book);
            }
        }
        file.close();
    }
}


void stock::fillPublishingHouse()
{
    std::ifstream ordersFile("orders.txt");
    std::ifstream wishlistFile("informAboutAcquisitions.txt");
    std::string detail;
    std::string value;
    for (PublishingHouse &ph : phVector)
    {
        ph.reset_Counter();
        ph.publishersDeliveryStock.clear();
    }
    if (ordersFile.is_open())
    {
        while(static_cast<void>(ordersFile >> detail), std::getline(ordersFile >> std::ws, value))
        {
            for (Book &book : booksStock)
            {
                for (PublishingHouse &ph : phVector)
                {
                    if (value == std::to_string(book.get_Id()) && ph.get_Name() == book.get_PublishingHouse())
                    {
                        ph.increase_Counter();
                        break;
                    }
                }
            }
        }
    }
    ordersFile.close();
    if (wishlistFile.is_open())
    {
        while(static_cast<void>(wishlistFile >> detail), std::getline(wishlistFile >> std::ws, value))
        {
            for (Book &book : booksStock)
            {
                for (PublishingHouse &ph : phVector)
                {
                    if (value == std::to_string(book.get_Id()) && ph.get_Name() == book.get_PublishingHouse())
                    {
                        ph.increase_Counter();
                        break;
                    }
                }
            }
        }
    }
    wishlistFile.close();
    for (Book book : booksStock)
    {
        for (PublishingHouse &ph : phVector)
        {
            if (book.get_Count() < 5 && ph.get_Name() == book.get_PublishingHouse())
            {
                ph.publishersDeliveryStock.push_back(book);
                break;
            }
        }
    }
}
//Вычитание заказов, сделанных до данного запуска программы
void stock::changeBooksStock()
{
    std::ifstream ordersFile("orders.txt");
    std::string detail;
    std::string value;
    std::string line;
    if (ordersFile.is_open())
    {
        while(static_cast<void>(ordersFile >> detail), std::getline(ordersFile >> std::ws, value))
        {
            for (Book &book : booksStock)
            {
                if (value == std::to_string(book.get_Id()))
                {
                    std::getline(ordersFile, line);
                    std::getline(ordersFile, line);
                    std::getline(ordersFile, line);
                    std::getline(ordersFile, line);
                    static_cast<void>(ordersFile >> detail), std::getline(ordersFile, line);
                    book.set_Count(book.get_Count()-std::atoi(line.c_str()));
                }
            }
        }
    }
}