Use the STL PRIORITY_QUEUE class with a custom type

This article describes how to define a Standard Template Library (STL) priority_queue template container adaptor class that uses custom (user-defined) data types.

Original product version:   Visual C++
Original KB number:   837697

Summary

This article describes how to use the STL priority_queue template container adaptor class with custom (user-defined) data types such as structures and classes. This article also describes how to order the priority_queue class members by overloading the left angle bracket (<) or the right angle bracket (>) comparison operators. This article also describes how to declare the priority_queue container class variables that contain the custom (user-defined) data members and how to access these variables in your program. The information in this article applies only to unmanaged Visual C++ code.

Requirements

This article assumes that you are familiar with programming with STL data types and container types.

Creating a custom data type

The priority_queue class is a template container adaptor class that limits access to the top element of some underlying container type. To limit access to the top element of an underlying container type is always the highest priority. You can add new elements to the priority_queue class and you can examine or remove the top element of the priority_queue class.

To use the priority_queue class with the custom (user-defined) data types, you must define a custom data type as shown in the following code:

//Define a custom data type.
class Student
{
    public:
    char* chName;
    int nAge;
    Student(): chName(""),nAge(0){}
    Student( char* chNewName, int nNewAge ):chName(chNewName), nAge(nNewAge){}
};

Specify QUEUE order

You can specify the order of the priority_queue class members by overloading the right angle bracket or the left angle bracket comparison operators as shown in the following code sample:

//Overload the < operator.
bool operator< (const Student& structstudent1, const Student &structstudent2)
{
    return structstudent1.nAge > structstudent2.nAge;
}
//Overload the > operator.
bool operator> (const Student& structstudent1, const Student &structstudent2)
{
    return structstudent1.nAge < structstudent2.nAge;
}

Create and access priority_queue variables with custom data types

The prototype of the priority_queue template class is as follows:

template <
  class Type,
  class Container=vector<Type>,
  class Compare=less<typename Container::value_type>
>
class priority_queue

Declare a priority_queue variable that specifies the custom data type and the comparison operator as follows:

priority_queue<Student, vector<Student>,less<vector<Student>::value_type> > pqStudent1;

You can use different methods of the priority_queue class such as push, pop, empty, and other methods as follows:

// Add container elements.
pqStudent1.push( Student( "Mark", 38 ));
pqStudent1.push( Student( "Marc", 25 ));
pqStudent1.push( Student( "Bill", 47 ));
pqStudent1.push( Student( "Andy", 13 ));
pqStudent1.push( Student( "Newt", 44 ));

//Display container elements.
while ( !pqStudent1.empty())
{
    cout << pqStudent1.top().chName << endl;
    pqStudent1.pop();
}

Complete code listing

// The debugger cannot handle symbols that are longer than 255 characters.
// STL frequently creates symbols that are longer than 255 characters.
// When symbols are longer than 255 characters, the warning is disabled.
#pragma warning(disable:4786)
#include "stdafx.h"
#include <queue>

#using <mscorlib.dll>

#if _MSC_VER > 1020 // if VC++ version is > 4.2
  using namespace std; // std c++ libs implemented in std
#endif
using namespace System;

//Define a custom data type.
class Student
{
    public:
    char* chName;
    int nAge;
    Student(): chName(""),nAge(0){}
    Student( char* chNewName, int nNewAge ):chName(chNewName), nAge(nNewAge){}
    };

    //Overload the < operator.
    bool operator< (const Student& structstudent1, const Student &structstudent2)
    {
        return structstudent1.nAge > structstudent2.nAge;
    }
    //Overload the > operator.
    bool operator> (const Student& structstudent1, const Student &structstudent2)
    {
        return structstudent1.nAge < structstudent2.nAge;
    }

    int _tmain()
    {
      //Declare a priority_queue and specify the ORDER as <
      //The priorities will be assigned in the Ascending Order of Age
      priority_queue<Student, vector<Student>,less<vector<Student>::value_type> > pqStudent1;

      //declare a priority_queue and specify the ORDER as >
      //The priorities will be assigned in the Descending Order of Age
      priority_queue<Student, vector<Student>,greater<vector<Student>::value_type> > pqStudent2;

      // Add container elements.
      pqStudent1.push( Student( "Mark", 38 ));
      pqStudent1.push( Student( "Marc", 25 ));
      pqStudent1.push( Student( "Bill", 47 ));
      pqStudent1.push( Student( "Andy", 13 ));
      pqStudent1.push( Student( "Newt", 44 ));

      //Display container elements.
      while ( !pqStudent1.empty())
      {
          cout << pqStudent1.top().chName << endl;
          pqStudent1.pop();
      }
      cout << endl;

      // Add container elements.
      pqStudent2.push( Student( "Mark", 38 ));
      pqStudent2.push( Student( "Marc", 25 ));
      pqStudent2.push( Student( "Bill", 47 ));
      pqStudent2.push( Student( "Andy", 13 ));
      pqStudent2.push( Student( "Newt", 44 ));

      //Display container elements.
      while ( !pqStudent2.empty())
      {
          cout << pqStudent2.top().chName << endl;
          pqStudent2.pop();
      }
      cout << endl;
      return 0;
    }
}

You must add the common language runtime support compiler option (/clr:oldSyntax) in Visual C++ to successfully compile the previous code sample. To add the common language runtime support compiler option in Visual C++, follow these steps:

1. Click Project, and then click <ProjectName> Properties.

   Note

   <ProjectName> is a placeholder for the name of the project.

2. Expand Configuration Properties, and then select General.

3. Select Common Language Runtime Support, Old Syntax (/clr:oldSyntax) in the Common Language Runtime support project setting in the right pane, select Apply, and then select OK.

For more information about the common language runtime support compiler option, see /clr (Common Language Runtime Compilation).