invoices example in modern c++

invoices_example.cpp

#ifndef INVOICE_H_
#define INVOICE_H_

#include <chrono>
#include <memory>
#include <ostream>
#include <string>
#include <vector>

#include "Customer.h"
#include "InvoiceLineItem.h"
#include "Money.h"
#include "UniqueIdentifier.h"

using InvoiceLineItemPtr = std::shared_ptr<InvoiceLineItem>;
using InvoiceLineItems = std::vector<InvoiceLineItemPtr>;

using InvoiceRecipient = Customer;
using InvoiceRecipientPtr = std::shared_ptr<InvoiceRecipient>;

using DateTime = std::chrono:system_clock::time_point;

class Invoice {
public:
    explicit Invoice(const UniqueIdentifier& invoiceNumber);
    Invoice() = delete;
    void setRecipient(const InvoiceRecipientPtr& recipient);
    void setDateTimeOfInvoicing(const DateTime& dateTimeOfInvoicing);

    Money getSum() const;
    Money getSumWithoutTax() const;

    void addLineItem(const InvoiceLineItemPtr& lineItem);

private:
    friend std::ostream& operator<<(std::ostream& outstream, const Invoice& invoice);
    std::string getDateTimeOInvoicingAsString() const;

    UniqueIdentifier invoiceNumber;
    DateTime dateTimeOfInvoicing;
    InvoiceRecipientPtr recipient;
    InvoiceLineItems invoiceLineItems;
};

#endif