Programmeringsmetodik, dt079g

Assignment for the laboratory work 4.

"The STL vector container, function objects"

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In this laboratory, it is necessary to develop a database as a part of some e-mail system, which could store and display the required data. First of all, class Email must be developed to keep information about a received letter: sender's name ("who"), date of receiving ("date"), and a "subject". These data members should be of type string. Class Email should contain a combined constructor, overloaded friend operator <<, as well as three friend function-like classes (structures): CompWhoDateSubject, CompDateWhoSubject, CompSubjectWheDate, objects of which will be used in the STL sort algorithm. Each of these classes has only one member function - overloaded function call operator () returning a bool value. Such an operator takes and compares two parameters of type Email, say, lhs and rhs, i. e., it's a binary predicate. The comparing algorithm should be a multi-level one (cascaded). For example, for the class CompWhoDateSubject, we need first to compare lexicographically "who" data members of the objects lhs and rhs. If we choose an ascending order, the operation

      lhs.who < rhs.who;

must return true. However, we should also consider a situation when

      lhs.who == rhs.who;

and compare the data members "date". If "date"s are equal too, we should compare the data members "subject". That is, if we have two Email objects:

        a("Anders", "2002-02-28", "lab 1"),  
        b("Anders", "2002-02-28", "lab 2"),

the call of CompWhoDateSubject returns true if a is lhs and b is rhs , i.e., a is less than b.

A nested if/else structure can be used for the multi-level comparing, or logical operators || and && may be used.

Secondly, a class MailBox must be defined, it should include as a private data member the STL vector container of type Email. The MailBox's combined constructor must initialize its member, calling vector's parameterized constructor of 1st type that produces a sequence of n elements. An access function enabling both vector's reading and writing should also be defined.

The class MailBox must also contain three member functions - SortWho, SortDate, and SortSubject, each of them sorts the vector by calling the STL sort algorithm with a corresponding binary predicate - a function object of class CompWhoDateSubject, etc.

In order to display the MailBox's vector in the main, a global function template Show should be created that takes a reference to a const vector's object.

In the main, create a MailBox's object with 3 vector's elements by default, fill in it with 5 Email's messages and test it.