Using the std::sort() Method

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Last year I was working on a database program that had to execute three queries, each of which had a couple of common fields and several dissimilar fields. The common data were starting and ending dates, and naturally the rows of data would have to be interleaved.

Purchase      Jan 1,  1990
Installation  Feb 2, 1990
Repair        Sept 23, 1991
Installation  Dec 10, 1993
Repair        Jun 4, 1996

Since there were three different but similar items, I needed a base class which I could derive from. The base contained the date stamp I would sort on, and for this example I’ve added a method to output text to an output stream:

// Base class we're using to provide a function with a

sortable value
class CHistoryItem
{
public:
     CHistoryItem(DATE& rTimestamp)
     {
          m_Timestamp = rTimestamp;
     }

     DATE& GetTimestamp()
     {
          return m_Timestamp;
     }

     // Indicate the item type as a string

     virtual   char* ItemName() = 0;

     // Output the item to a stream

     void OutputToStream(std::ostream& os)
     {
          os << ItemName() << "\t" << m_Timestamp << "\n";
     }

protected:
     DATE m_Timestamp;
};

Once the base class was defined, I derived the three history items from it:

// Repair history entry

class CRepairItem : public CHistoryItem
{
public:
     CRepairItem(DATE rTimestamp)
          : CHistoryItem(rTimestamp)  { }

     // Indicate that this is a repair

     char* ItemName()    {    return "Repair: ";  }
};

// Installtion history entry

class CInstallItem : public CHistoryItem
{
public:
     CInstallItem(DATE& rTimestamp)
          : CHistoryItem(rTimestamp)  { }

     // Indicate that this is a installation

     char* ItemName()    {    return "Install: "; }
};

// Purchase history entry

class CPurchaseItem : public CHistoryItem
{
public:
     CPurchaseItem(DATE& rTimestamp)
          : CHistoryItem(rTimestamp)  { }

     // Indicate that this is a installation

     char* ItemName()    {    return "Purchase: ";     }
};

Ok...so we have our class definitions, now its time to create a vector which will contain our objects. Since I didn’t know what know prior to the query is performed what to expect, we have to dynamically allocate objects. Since I’m a bit lazy and forgetful, I tend to free the objects I allocate in my vectors in the vector destructor.

// A vector based class to contain our history items

class CHistoryVector : public std::vector<CHistoryItem*>
{
public:
     // Make sure that the history items are de-allocated so we don't leak

     ~CHistoryVector()
     {
          for (iterator pItem=begin(); pItem != end(); ++pItem)
               delete *pItem;
     }
};

Now that we have everything set up, we need to provide a way of specifying how we want to sort the data. Now typically I only need to sort my data one way, but to show how the sorting works, I’ve created two sorting methods. Again, I could have put more work into the CHistoryItem class and given it an operator < method, but then what I’m doing might not be so obvious. Basically the sorting classes must have an operator () method that returns a bool and is passed two elements from the vector by the sort() algorithm. It is your job to return true if the first item is to placed before the second item in the sorted output, false if it is after the second item.

// Ascending date sorting function

struct SAscendingDateSort
{
     bool operator()(CHistoryItem*& rpStart, CHistoryItem*& rpEnd)
     {
          return rpStart->GetTimestamp() < rpEnd->GetTimestamp();
     }
};

// Descending date sorting function

struct SDescendingDateSort
{
     bool operator()(CHistoryItem*& rpStart, CHistoryItem*& rpEnd)
     {
          return rpStart->GetTimestamp() > rpEnd->GetTimestamp();
     }
};

All we need now is something to create our vector, sort it, and then do something that would indicate to us what the output is. Here is a simple console application which can be used to test our example:

// Stuff a vector with data & then sort

int main(int argc, char* argv[])
{
     CHistoryVector HistoryVector;

     // Put two repair items in the vector

     HistoryVector.push_back(new CRepairItem(2*365.0));
     HistoryVector.push_back(new CRepairItem(5*365.0));

     // Now stick three installations items in the vector

     HistoryVector.push_back(new CInstallItem(3*365.0));
     HistoryVector.push_back(new CInstallItem (6*365.0));
     HistoryVector.push_back(new CInstallItem (4*365.0));

     // Finally we need to add a purchase item to the vector

     HistoryVector.push_back(new CPurchaseItem (1*365.0));

     //==================================

     // Sort the items in ascending order

     std::sort(HistoryVector.begin(), HistoryVector.end(), 
               SAscendingDateSort());

     // Now show the results!

     std::cout << "Ascending Sort\n";
     for (long lEle=0; lEle < HistoryVector.size(); ++lEle)
          HistoryVector[lEle]->OutputToStream(std::cout);

     //==================================

     // Sort the items in descending order

     std::sort(HistoryVector.begin(), HistoryVector.end(), 
               SDescendingDateSort());

     // Now show the new results!

     std::cout << "\nDescending Sort\n";
     for (lEle=0; lEle < HistoryVector.size(); ++lEle)
          HistoryVector[lEle]->OutputToStream(std::cout);


     getchar();     // Pause for a response

     return 0;
}

So how does it look when its run?? Where here is the output:

Ascending Sort
Purchase:       365
Repair:         730
Install:        1095
Install:        1460
Repair:         1825
Install:        2190

Descending Sort
Install:        2190
Repair:         1825
Install:        1460
Install:        1095
Repair:         730
Purchase:       365

Obviously the dates haven’t been converted to a nice string, but we could obviously add this if needed...the main point is to note that the first set of data is sorted in ascending order while the second set of data is in descending order.

Paul's company is ADOPro.com.